Overall cost is greatly dependent on your choice of materials, and your choice of materials is highly dependent on your personal skills and how comfortable you are working with said materials (not to mention where and how you acquire these materials, as some places charge far more than others, and occasionally you can find good deals of people practically giving stuff away).
The two main methods are pepakura/fiberglass, and EVA foam. The Pepakura method revolves primarily around working with card stock (heavy paper) and can be tedious with the need for printing out, cutting out, folding, and then gluing together all the pieces. Essentially it's like constructing a 3D puzzle by matching the numbers printed along every edge of every piece, and depending on the model and how it was unfolded (and how detailed it is overall) that can mean anywhere from a couple dozen pieces with about 150 edges to be glued together, to a hundred pieces with 700+ edges to match up. Again, that depends entirely on how detailed the base 3D model is and how that model was unfolded for printing. Once assembled, the finished piece will have to be lightly coated with resin to harden it, then reinforced with fiberglass on the inside and smoothed over with bondo on the outside. Details can be lost in this stage that will have to be sanded or dremeled back out again.
The foam method revolves primarily around the use of 1/4 inch foam mats, such as the interlocking floor mats often found in automotive and sporting good stores, with finer details being added with the much thinner sheets of craft foam (often referred to as "foamies"). This also involves hot glue (the most recommended bonding method) and you will have to seal the finished piece before primer and paint can be added (generally using PVA glue, melting and painting on hot glue, and/or Plasti-dip or any other available rubber spray or brush on coating. There was also a method posted recently (can't remember who but I believe it was by EVAkura) with another method of sealing that provides a smoother surface to work with.
Going by rough Walmart prices, you can get a ream of 150 sheets of cardstock for $6, or a pack of foam mats (usually 16-24 square feet total in 2 x 2 mats, depending on if they have 4 or 6 in a pack) for $15-25 depending on your area. Just from those figures cardstock might seem like the cheaper option, but remember not only does it take more material to build up the same thickness, but you'll also have to buy fiberglass mat ($6-8 for roughly 8 square feet) as well as resin (depending on quantity, $10-20 per can). And if you're working with fiberglass resin (as opposed to the non-toxic alternative, Aqua Resin) you will also want to pick up a respirator to protect you from the fumes.
All in all foam is probably the safer and overall easier than fiberglass, but it does take a bit of extra effort to get the same level of detail. It's also lighter to wear. Fiberglass is slightly more forgiving of mistakes, however, as it takes to sanding if you've got too much and bondo if you've got too little, whereas foam isn't quite so simple and straight-forward when it comes to "patching."
As for your budget, I honestly can't say for certain as I have yet to complete a full suit or even a helmet yet and I've mostly been using materials already lying around the house. I would venture that overall $200 would be a rough starting price for at least a basic suit. Some builders spend more to add electronics, such as LED lights, cooling fans, or special effects. Some also expand their material list (and thus overall initial cost) by taking it a step further and once they've completed their build, they create a mold and then cast the piece in their chosen material. Again, higher overall initial cost, but so long as you are happy with the look and fit of the piece, if something happens to it you can just cast a replacement for less than the cost of rebuilding.
So I guess the short answer is there is no short answer to this. It comes down to what you are comfortable working with and how far you want to take it. You definitely can build a full suit yourself for less than the $800 you were planning to spend on one. I don't know if you were looking at something store-bought or paying someone else to make the suit for you, in which case a large portion of that price is more to cover the time put into the suit than the material cost. A high-end suit takes time and effort to do right.
Which brings us to your second main question: how long does it take? The usual quip is "how much time do you have to put into it," meaning how much free time you have per day to dedicate to working on your project. REgardless of which method you choose, you'll need time to print and cut out the templates, assemble the base piece, seal/reinforce it, fine tune and add detail, possibly seal/reinforce again, prime it, add base paint, add detail paint, and then top it off with a clear coat to protect the paint. For a Halo 3 style build, the paint stages won't have to be too intricate, as each individual piece of armor is essentially just one solid color (although you may want to add weathering and/or "battle damage" effects) compared to Reach and Halo 4 where pieces could have 2-5 colors. Again, foam generally is seen as the faster of the two methods, as you don't have to deal with fiberglass curing times and whatnot, and those with a practiced hand have been able to construct a full piece or more in a day (smaller pieces like boots, shoulders, and gauntlets obviously take less time than chest armor and helmets).
As for the buying and selling, there IS a classifieds section in which such arrangements can be made, but I don't know the status of that right this moment, what with the site changing hands and facing a potential overhaul. In the old system you needed to achieve "infantryman" membership status, which while some exceptions were made usually involves being a member for at least 6 months and having over 100 posts (and not just a bunch of one-liners or multi-posts). I don't know how or even if that system will be changed with the new administration. If it is available to you now, it will appear in the blue navigation buttons at the top of the page and you can browse for people selling (WTS: ____) or post what you are looking to buy outright (WTB: ____) or if you are looking to arrange some sort of trade/barter for it (WTT: ____). If it's not there, then the old system is still in place, in which case it would not be permitted for you to post a thread elsewhere asking to buy anything. If you come across someone else's project in the forums and you like their work you can try sending them a private message to see if they would be interested in commission work, but do so with the understanding that the nature of this group is to help people along the path of building their own rather than building for them, so you might not find very many who are willing to take on a commission. Also understand that there are certain things that come with commissioning someone else to do armor. For one, the price vs doing it yourself obviously will be much higher, as you not only need to pay them for their time and effort (as well as materials, obviously) but also for shipping the finished piece to you. Also you would be expected to provide a list of certain measurements to ensure a proper fit, or else have to send them a duct-tape dummy of yourself (there are numerous tutorials to be found around the net for how to do this if you don't already know how). Even with a dummy or measurements, there's still the possibility that a piece won't fit comfortably, and each person will have different terms for dealing with this possibility ranging from "I take responsibility for the miscalculation and will send a replacement" to "I put time, effort, and money into making that piece so I still expect to be paid something for it," one of many details you will want to iron out with the other person and come to a mutual agreement before moving forward. All in all, commissioning can be just as, if not more complicated compared to just doing it yourself. And doing it yourself gives you the pride of accomplishment knowing you built it with your own hands.
One of the next likely questions will be "where do I find the models/templates to build from?" For that, seek out MissingSpartan7's database thread for a link to the 4shared database that has a large number of available models to choose from (you'll find this database referenced to many times within the forums as it is where all community-made files here in the 405th are eventually gathered into one place). I know there's one for Reach and Halo 4, so I assume there's one for Halo 3 as well, but look for a "Assets Release" thread if you want to work from a model pulled straight from the game disc itself. This is a bit of a gray area practice, as it walks the fence line of potential copyright infringement and whatnot, so more often than not people rely on the community-made models instead, particularly as many of the armor details are "painted" on through texture maps rather than actually being rendered in-game. For example, on the Halo 3 Rogue helmet you'll find long nubs on the cheek plates, bolt/screw indentations, and other such details when you look at it on screen or even on a toy, but the model pulled from the game is entirely smooth, so these details have to be added by hand during the build process.
