Most fish shops actually make very little profit on each fish sold.
Shipping obviously costs a lot, then there's the paperwork involved, and someone has to be paid to handle the fish once they get to the shop. Then there's the cost of quarantining livestock, which means you're feeding and warming and filtering fish, with a certain risk that some will die. Medication, UV, etc all costs money. Even when on sale they're still using up resources. You have to feed, warm and filter the fish, and you also need to pay for someone to catch the fish and put it in a bag for you. And don't forget the cost of running the shop in the first place! Rent, paperwork, bank charges, taxes, etc.
Those fish that will make a profit are the bread-and-butter species. Really, the smaller and cheaper the fish, the more profitable selling that species across a year will be. You might make pennies per fish, but you'll sell hundreds or thousands of them. By contrast, things like SPS corals aren't especially profitable at all, given how difficult they are to keep alive relative to, say, tank-bred pond fish, which tend to be highly profitable.
Unusually cheap wholesale fish might not necessarily be collected or handled in the best possible way. A cheap fish caught using cyanide isn't really a bargain if the thing dies six weeks later.
So while it's easy to think pet stores are scalping you, they really aren't. Sure, there's variation, but what you should be looking for is healthy livestock that's been properly handled and quarantined. If that means that fish costs £30 instead of £3, does that really matter if it means you have a reef fish that's genuinely healthy and likely to live a normal lifespan in captivity?
Cheers, Neale