If you're selling directly to a retail store, if they buy animals from breeders at all, they'll tend to pay you 1/3 - 1/2 of what they sell the animals for. So (at least in my part of the country) you might make $15-20 on a normal baby ball or corn, maybe $10-12 on a leopard gecko, maybe $20-25 on a crested gecko. A lot of pet stores will only want to buy one, two, or three animals at a time, though, not whole clutches.
(If you're selling to a major wholesaler -- say, LLL or Calzoo or whomever -- you'll get tend to get about half of what a petstore would give you, so maybe $10 on a baby ball or $15 on a crestie, BUT they'll usually buy as many as you can produce.)
As far as stores, a good strategy is to ask "well, what do you normally pay your wholesaler/supplier for this animal?" and then name a slightly cheaper price. If they normally pay $35, say, for a butter cornsnake, offer yours to them for $30.
If they don't want to reveal the number, ask for half of what they're selling it for -- but be willing to take a third. If you start right off giving them the 1/3 of their retail price number, though, you might be underselling yourself if they'd happily give you half. Just like negotiating salary in a new job, it's best to get "the other guy" to name a price first.
I wouldn't bother looking at kingnsnake/fauna prices: most pet stores don't deal much in the morph market, and prices do vary a lot regionally. But know the going rates of the animals you're selling, at all the local pet stores near you (and also the going rate of them at the "big box" chain stores.)
