How much is an iPhone 4s supposed to cost?

For pay as you go. 5 is to expensive for me.

I would always get on contract, then it doesn't cost much at all, 4S is free on contract. Never looked at pay as you go plans.
The cheapest service I think you can get is probably Straight Talk, they are around $45 which seems to be by far the cheapest monthly plan. The service probably isn't as good as the big carriers like verizon, sprint, etc, but its the cheapest option.