SS SCHGD

The G/f has a Cobalt LS and has the base CD player without the MP3 option, The dealer told me that they could charge us 800$ to replace it when we bought it, but since we are haveing problems with it and went to a different dealer, they said that we might be able to trade in the old one and pay the difference for the MP3 one, and they could install it....Is there anyone that can look into this for me and maybe give me a idea of the actual cost? I want to keep the Stock look (we'll she does, so I can't go aftermarket yet) so thanks for any help.