I'm broke and payday is still a few days away-- I decided to use foam)
Foam, although being very popular, and in some ways a little easier to do, generally is more expensive than Pepakura. If you're like some of us (myself included) who aren't yet in a great job with plenty of money to throw at your project, I'd suggest using Pepakura before foam, simply because of the cost.
My biggest overall tip though (more for pep than foam) is that foil is your friend. I've found that you can use foil in so many ways in modeling it's crazy. From a drip sheet, to a disposable resin/ glue bowl, it really is a versatile, but cheap material.
Another cost tip is check Walmart! I've found stuff at Walmart I really didn't expect to, but you never know what they'll have in stock. Before buying something at a specialty store such as Hobby Lobby, check Walmart :]
I do agree with Carpathia though. In modeling like this only 20% of what you learn will be from forums, videos, and people. The remaining 80% will come from trial and error. Don't expect your first build to be your final, and always anticipate the fact that you may spend hours, even days on a certain piece then have to throw it in the trash and start over. Although infuriating at times, this is well worth it in your final product.
One final tip, more of an opinion of mine, is
don't go overboard with battle damage! I see some builds that just go way over the top, and people get the feeling that every single little piece they made must have some form of damage on it. Sometimes less is more. I don't want my armor to look like it was thrown into a giant blender for ten minutes, so rather than scrape and scuff every single edge, perhaps focus on one or two very specific and detailed battle damage points. If you haven't already, head over to the Elite forum and check out Cereal Kill3r's Halo build, and the damage he put on it, it's pretty impressive and doesn't look like it was kicked down a hill.