How much would I get at gamestop for trading in a PS3 slim to buy the newer PS3?

I bought my PS3 slim last october, so it hasnt even been a year yet, and it looks practically brand new it is a 320 GB model and I just wanted to know how much Gamestop would give me for it ( In store credit so I can get more money plus the bonus I get from my power up rewards pro membership) or is Sony going to do something where they will accept older slim models and give people newer models