I bought a 3G iPhone from a friend for 50 bucks, got a go phone sim card, pay 60 USD a month for unlimited talk/text. I also unlocked it on AT&T network (unlockit.co.nz view it from safari) so I can buy data packages if I know I'll be needing it.
Data packs are fucking expensive I think, 25 bucks for 500MB. BUT It does rollover as long as you renew before it expires. I'm currently sitting at 1GB.