HOW MUCH DO YOU REALLY PAY FOR AN IPHONE?

I’m formulation on shopping an 8gb iphone. I am already a patron for At&t and wish to know a couple of things.
1) can i put my old sim label in to the iphone and still make use of it?
2) How most will i compensate for a brand new 8gb iphone with no contract?
3) will i have to compensate some-more per month than what i’m profitable now?
4) if you have an iphone, is it value it?
* where could i buy an iphone with no contract?