Pepakura is a method of making high-quality costumes and props out of paper. 3D models of items are sliced into flat pieces and laid onto a grid, then printed onto cardstock. The pieces then are cut out and assembled. After this, the model is fiberglassed to harden and strengthen it, smoothened out with bondo resin, and painted. The end result, if done right, turns out like this.
(File used is available in the Pepakura Database.)
While a vacuum-molded costume often costs thousands, a good quality pepakura costume can easily be made for under $800. However, it is difficult, and you may need to try several times over before you get a part right. Pepakura is also time consuming, and simply assembling the paper pieces can take alone can take weeks. But the payoff is well worth the time, money, and effort.
The 405th Infantry Division, though primarily Halo-oriented, is a great resource for anyone looking into making a pepakura costume. Costumes from any franchise are allowed to be displayed, the members are generally understanding and helpful, and there are plenty of great tutorials.