i wouldnt really worry about the money,the main point is to find them a good home.i'd say free to a good home.the bigger they get,the harder it will be to rehome them.
trouble is samxx,in a few months time,you'll end up with a couple large fish that no one will want.i try the free to a good home,or,try taking them back to another fish shop and see if they will take them???
have u tried offering them in paper?? diamond freeads, lfs notice board perhaps?? an aquarium magazine?? b lil harder to sell em on here cos ppl from all over put em in paper and more local ppl see advert so more chance of selling em.