IPhone 4 value vs iPhone 4S?

I'm wanting to trade my AT&T 16 gigabyte iPhone 4 for an AT&T 16 gigabyte iPhone 4S in the same condition. I know the iPhone 4S is worth more than the 4 but by how much? For answer if you're just guessing because I can do that all day. Please only answer if you know what you're talking about. I just want to know how much cash to add on to make it a fair trade.

If I was going to upgrade from 4, then I would go to 5C. 4S is better but its not huge over 4.