PetsUnlimited here in Nova Scotia is sort of odd. Anything small, like barbs, tetras or corys, and you'll get a credit for the full value of the fish at retail price. I've also received full credit for some mid size fully grown cichlids - I got $24.99 for my 7" Jack Dempsey when I returned him and $14.99 for my 5" Severum.
Large cichlids (8+ inches) and large plecos are handled differently for some reason. I traded in a 13" common pleco a couple weeks ago and was told the store policy on plecos is to replace it with a baby pleco. I returned to the store the next day and they slapped a $49.99 price tag on it. Needless to say I was not impressed by that. A friend of mine recently traded in a pair of breeding Jags (approx 11" male, 7" female) and received $9.99 each for them. He visited again within the week to notice the pair listed at $89.99.
I'm trying to figure out what I'm going to do down the road when I move out of my apartment, I've got some fish that I just barely have enough tank space for and some oddballs that don't quite fit in anywhere. I can't even imagine what they'll offer for my semi-rare, exotic fish. Haplochromis Ahli breeding pair that I bought for $70.00, or my Polypterus Senegalus pair that cost me $50 for the pair as juveniles but eat about $10 worth of food a week. Something far too low would be my guess
Oh well, I'm more excited about planning my new 125 or 180g tank when I finally graduate and settle down. Torn between two ideas at the moment, first option is arowana's and some marbled or motoro rays; second option is a fahaka or mbu puffer all by itself. If I could ever spend the money on it, M. Tigrinus would be awesome as well