get reference pics first, then look at what you want to make it out of, and then find prices for all the stuff you want to make your suit out then make a bodycast(not necessary, just good fitting if you do) then sculpt all your stuff on the bc then cast it and go from there. any questions go to http://www.bioweapons.com/Costuming.htm and thats pretty much it.
Try making your own. I'm using cardboard at the moment, and its going along fine. I'm gonna go over it with fiberglass resin later probably. So far I didn't spend any money at all and right now I have a wearable belt, and thigh armor. I'm gonna try to detail them a little more, though. Check out Monstermaker's armor and Ben's scale drawings, Those are helping me alot.