Depends on the store/mood of the owner. 1/3 retail price would be the maximum you could get, unless the fish is worth (retail) over maybe $20 to $25. After that, you might be looking more towards 1/2 retail max. Depends on the store's policy as far as margin per cost goes. As the price goes up, margin usually goes down.
Most retailers can get, say, a 3" comet for ~70 cents, and they will turn around and sell that fish for $3 to $4. However, to stay competitive and move more of the expensive stuff (over $20), they reduce the margin to ~100% instead of ~300%.
Your best bet is to find a hobbyist near you willing to buy. The reason you won't necessarily get those same prices at a store is because they don't know what your tanks are like, there is just too much ambiguity for them to deal with, so they may refuse to buy or offer very little.
