Right now Cingular is offering two phones, one of which comes in two varieties. The Nokia 6010, the old stanby of GSM that is a great basic phone, if basic can be termed to a phone capable of web access, MMS, text messaging and voice calls (probably can), is $30. Surprise, so is the Samsung c207, a phone more likely to stay on Cingular (not as easily unlockable) but small and neat enough that you might just have to get the cable and unlock it for T-Mobile as well.
If you want the c207 you have two choices: buy it new for $30 or buy it refirbished for $30. What's the difference? The new one had a $30 mail-in rebate that you need to redeem to get that price. The Nokia is new.
Nice thing about all GoPhones is that if you go to DealKing (I think DealKing.com) and follow the directions somewhere there you can get $30 back from them for getting a GoPhone, or that's the last I heard. So you can give it a try and if it works you've just become the proud owner of a free prepaid phone, a comcept pioneered by STi Mobile ;).