We seemed to do pretty well last season as rookies with a home made field on our back porch:
http://www.flickr.com/photos/21178067@N02/3533703569/
Using Walmart exercise mats ($17per six mat set), goal and field elements made by a supportive mechanical engineer from aluminum and plywood (probably about $50 worth), hockey tape (and later first aid tape, about $2) and some corrugated material from Home Depot ($8), and no perimeter, we managed to win the Inspire Award at our first qualifier and ranked 9th out of 24 in the state championship, finishing out with a Finalist Alliance Award.
By the end of the season, we'd gotten the full complement of mats, and I think the whole thing cost us about $175.
We'd really like to have a perimeter this year and may or may not rough in all the elements again ourselves, depending on what the challenge is. But I don't think it's as necessary to have an "official" field, so much as to have everything accurately placed on the field, as per the design drawings.
Terri
www.TeamDuctTape.com