Look at the bottom of this page on AndyMark's web site:
http://www.andymark.biz/offseasonfield.html
It points to
a complete design package and says the raw material is about a thousand dollars.
The TechnoKats built ours a few years ago, adding game-specific parts every year (including the third gate for Breakaway). We bring it to IRI to be the practice field in the pit area. It was used as the playing field for the first C.A.G.E. offseason competition. It works great, and it's no great hassle to disassemble, transport, and reassemble it.