The experience I have had is that they are usually $20 for 50-60 minutes. Also you can get them at Best Buy,Target,Walmart,and other places like that. Reason why that would be better then say 7-11 is that you can get accessories and usually a better phone.
I was looking at BB the other day and this was their deal:
Nokia 5160 with AT&T service: $150, includes setup fees (regular AT&T is $99 + $25 activation).
The service is nationwide, no roaming or long distance. The rates were the strange part: the 30 minute card was cheaper than the hour card, and the 15 minute card was even cheaper (on a per-minute basis). I believe the rates were $12 for an hour, $5.50 for a half hour and $2 for 15 minutes. I know it sounds strange, but the salesman used the register and actually scanned each one for the price.
I might have those prices wrong. I was just over at Office Depot and they actually had the AT&T cards out. $39.99 for an hour, $69.99. Not quite the same deal. So either the guy at Best Buy was an idiot, or. . .I dunno. . .
They also offer the Ericsson 588 (not sure of the letters). Didn't catch a price on that one, though. It's usually cheaper than the Nokia.