I shoot, less than I would like, at Boone, IA- Boone Sportsman's Club. We get together once a month and pay $5 a gun. It is a pretty laid back group that just likes to have fun and shoot.
There is also a group at Ames, IA Izaak Walton
If you join this group on Yahoo it covers both: http://sports.groups.yahoo.com/group/CentralIowaIDPA/
Pretty sure there is also a group that shoots at Osceola, Ia, but I don't know anything about it...
As far as costs, for Boone there is not a "join" fee. We take care of some basic clean-up for the range and they don't charge us for it's use. We pay $5 a gun to shoot the courses for the day.
You will need eye and ear protection. Obviously a hangun and two to four mags with holster and mag holders. A concealment garment(jacket works).
Other than that, whatever else you feel you need: range bag, mag loaders, cleaning kits, the list goes on.
Cost for ammo is whatever you want to spend. If you reload and cast then that is pretty cheap. Basic store bought ammo is pretty common and premium ammo is going to cost more.
I would check out the site here: http://www.idpa.com/
And, the AR15.com threads here: http://www.ar15.com/forums/forum.html?b=9&f=15