How much do you really pay for an iphone?

I’m plotting on buying an 8gb iphone. I am by now a customer for At&t and want to know a few things.
1) can i place my ancient sim card into the iphone and still use it?
2) How much will i pay for a new 8gb iphone with no contract?
3) will i have to pay more per month than what i’m paying now?
4) if you have an iphone, is it value it?
* where could i buy an iphone with no contract?

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