What is the monthly cost of the iPhone 3G S?

Answer 1

Your standard AT&T voice plan fees apply, plus the $30/month unlimited data plan for the iphone. This is the same fee that you would have for a Blackberry. You can also add various text messaging plans. The minimal additional fee over a standard phone though is $30 per month.

Answer 2

This will depend very much on where you live and what network/carrier you use. In the UK it is £40 per month to have an iPhone on a monthly contract over 18 months however this is just one example.
That depends on the network you choose to take the handset out with. Contracts in the UK start from £35 per month with roughly an up front cost of £100