I'm in the process of building one also, using 5.1 acres and I found alot of scrap culverts...18", 24", 36" and a few big ones(for table top)... They're black plastic, very light and very strong.
I seen a road crew 4-laning a hwy and I stopped and talked to the Super and he was happy to give me all I wanted...He wouldn't even take $$$, what a cool guy!
Hopefully that will save alot of Dirt.
The way it is now it will be about 3/4 of a mile long, 10' wide, two table tops(one will be 60'), one step up, one small woop section, and a butt load of doubles and various jumps coming into and going out of corners...
I'm also gonna try the logs and stumps for some of the track...
Goodluck, Moto