not much I would think, they tend to be pretty tight about that. I saw an Oscar that size @ my LFS and it was selling for 9.99, so I would think not much. However, each store is different and you might be able to get more credit for him. GL
yeah there are tons of people trying to get rid of larger oscars, so i owuld think unless he is like an uncommon color morph or something you wont get much for him, i would try selling him yourself
yeah my LFS rarley buys large oscars and when they do get one they usually dont pay anything for them, plus they dont keep the small ones in stock because they will just end up geting them back when they out grow the owners tank anyway.