I have sold my angel babies from anywhere from $2-$5 to my
lfs. But, it depended on the variety and size and their current stock. For most guppies, plenty of stores won't even take them free because of their ready availability.
Stores are indeed more likely to pay for things that are uncommon, difficult to breed, or expensive from their suppliers. I got $4 per rcs at one
lfs for awhile. Eventually, it was lowered to $2 per rcs, but even that is still awesome. However, that was a rare instance in a small town. Try calling around and see what they say.