Just ran across this site, unbelievable realism! I've worked a bunch with foam board, I could get a 20 x 30 inch piece of 3/16 foam board at the dollar store for a buck a sheet, just had to remove paper from either one or both sides, some really cool stuff to work with for dioramas and even models in parts. It's not like regular Styrofoam, that won't work. Takes some practice, but it's fun to work with and the price of buildings is crazy!
http://www.009.cd2.com/members/how_to/nouaillier_a.htm
http://www.009.cd2.com/members/how_to/nouaillier_a.htm