ODST has much less armor to put together, so the overall price of making a set of armor is somewhat offset by the raw materials. I'd estimate about $100, $150 max, for the main costume expenditures. Probably less than that. You'll need 110# card stock, resin, fiberglass cloth and/or mat, 120-400 grit sand paper (600 might be a little over board), an "exacto" knife to cut and score with, a metal ruler for creasing, BDUs, boots, I'd recommend some under armor, velcro and webbing for holding everything up, and something for the visor (either a CD spindle case, as some have used (?) or a motorcycle visor). You might also want to look into extras like a fan system for the helmet, and possibly a molle vest if you can get it cheap to incorporate into the ODST vest. Hot glue to put the helmet together with, or maybe a cyanoacrylate (super glue).
The subdued urban camo pattern that they use may run around $50 for a set (including S&H) depending on your source. I've got a set of rip-stop coming from bdu.com (mine's a little over $60 'cause it's rip stop), and will review them once I get them. The visor can run from 10 to 30, depending on what you get and from what supplier. I'll be looking for a blue mirrored motorcycle visor once I figure out the pepakura scaling, as I've got a clear regular one from my actual helmet I can use as a stand in.
Starting from a Pepakura basis, (the files are
here ) you'd resin the outside, mat and glass the inside, then sand to round it off and add in detailing as with the typical fiberglass tutorial.
I've got the pepakura print out of the helmet in process to figure out if I sized it right (I don't have a digicam to show progress), then I'll do the "vest" and knee/lower leg armor once I get that figured out.... I'm also trying to piece together the deltoid armor plates and a few other details from the full pepakura. 90% of my problem is just being able to pick out the edges of the pieces in the 3d or 2d displays then getting everything to fit.
Right now I'm running into issues with having time and space to work in. With all of this, the most costly and irreplaceable commodity is time.