There is a pretty simple relationship between price and easy to breed. The easier it is to breed a fish, the lower the price. Ok, so it's not quite as simple as that, since there are some fish that are near impossible to breed, but you can buy the wild parents for between £5 and £10 ($8-15), but nearly no one is breeding them - they aren't easy to breed, but plenty of them in the wild, and pretty hardy.
However, because the selling price of fish increases less rapidly than the difficulty in breeding them, the best way to make money is to breed the easiest to breed fish -
- if you are lucky, you can make a bit extra money by breeding one of the less common forms (albino, long fin, piebald, super-red, etc).
The other factor of course is the fact that the bigger the price tag, the fewer potential customers you get - more so if you want to sell a large number at once - there are only so many shops that can pay out cash/shop credit for fish worth £1000/$1500, where the number of shops happy to give you £50-100/$80-150 in credit are much more likely. And most shops won't think twice about stocking fish that sells for £5/$8 or so, where stocking more than a couple of fish that retails at £50-100/$80-150 each is someting only a few shops will do.
You can of course sell fish privately - this, like selling a car privsately, is a matter of "money vs. convenience". If you sell two bags of juvenile fish to the shop for £0.50/$0.75 each, you get, in one go, enough money to buy a bit of food or some such. But you could make more money by selling them yourself to customers for £3/$5 each. However, you are unlikely to sell more than one or two at a time, and you have to accept people coming to your house, rather than just making one phone call to the LFS asking if they are happy to take some of your fry...
For more expensive fish, you will probably HAVE to ship fish (and sell privately), because the number you can sell locally is simply too limited.
And, as Suckermouth says, if you want to make money, there are certainly better business models than breeding fish - plecos or otherwise. But the easier to breed, the easier they are to contribute some money back to the drain tat is fishkeeping... ;)
--
Mats