How much is an iPhone 4s upgrade?

I have the iPhone 4s and now it's time for my upgrade. How much is it?

Why would you want to buy another 4S if you already have one? Or do you want us to try and telepathically acquire the next phone you would like.
Upgrades are much cheaper but it depends on the phone, carrier, method. A quick look on ATT told me that upgrade eligible iphones 16g with att (2 year commitment) cost;
6s+ $299
6s $199
6 + $199
6 $ 99
5s Free

Why 'upgrade' to the same phone?

Check with your carrier concerning the details of your upgrade.