How much to upgrade my iPhone?

So I'm on Verizon and I have a 4s. I got my phone in December of 2011 and my brother used my upgrade to get his new phone in June of 2012. I dropped my phone from the top of a set of stairs onto a concrete floor. The phone is working and there isn't a crack (thank you Otterbox) but it's very glitchy. It's getting to where it'll stop between screens and it'll take a few minutes of turning it off-and-on for it to start working again a few times a day. It doesn't bother me but I want to get it sorted before something happens and my phone stops working all together.

How much would it cost to upgrade if I'm on a 2-year contract? I'd either switch to a a working 64-gig 4s or 64-gig 5. I tried researching it but I got confused because every site said something different about switching.

There's no special price for the iphone, the $200-300 price is the lowered upgrade cost. The iphone with out a contract or with out an upgrade is a lot more expensive, more like in the $600-700 range. So if you have an upgrade that means you are going to resign a new 2 year contract and you get the special $200-300 price.