How much should i pay monthly for an iphone?

I bought an iphone 4s and i want to pay monthly but i want something cheep can i buy cards by 10 dollars or something like that?

Best deal right now is from Straight Talk prepaid, unlimited talk and text with 2 GB of data for $45 a month final cost, no fees, no contract. Best value with the plan comes when you buy it in 3 month or more increments. If you buy it month by month it costs $50 in the end (still good but the 3 month package saves even more money). I tried for 3 months to get a $10 a month prepaid plan to work on the iPhone but was unable to do it. But considering that the equivalent plan from AT&T costs $90 a month, I sure don't have any problems with $45.

I use simple mobile $40 unlimited plan.
you would need an unlocked iphone