New AT&T customers who pass a credit check know what they will pay for a new iPhone: $99 for an 8 GB iPhone 3G, $199 for a 16 GB iPhone 3GS, and $299 for a 32 GB iPhone 3GS.

AT&T also charges a $36 activation fee. The firm requires a two-year contract, and a pretax minimum $40 voice plan, along with a $30 data plan. Text messages are not included in these minimum monthly fees.

If you’re an existing iPhone owner, the pricing gets more complicated. Subscribers who are still within a two-year contract may be eligible for the prices cited above. AT&T subsidizes handsets, absorbing the difference between what a handset maker like Apple charges it, and what the firm passes along to a subscriber. With iPhones, that difference is believed to be from $200 to $400.

AT&T recoups its subsidy from you as part of its monthly service fees, which also must pay for the capital and operating costs of its network, and (shareholders insist) produce a profit, too.

Last week, AT&T said that customers who paid at least $99 per month per line would be allowed to get an iPhone at the cheapest price 12 to 18 months into their contract…