WalMart. 6x3' (ish) tables are $40 a pop.
http://www.walmart.com/ip/Mainstays-6-Foot-Long-Center-Fold-Table-Multiple-Colors/13269097
Ive got one, and going to pick up a second this week. Makes a great painting table by itself, and two side by side will get you a 6x6' area and handily fold up and can be put away.
Next, get to your local hardware store and score yourself some foamboard. I bought a 8x6' piece, an inch thick, for twenty bucks. Cut that down to fit the two tables. Dig trenches and terrain into it. Buy additional foamboard to build up hills. Add addition terrain as needed.
150 bucks. Done.