Let's think about this statement for a bit. Go to a LFS and look at their snail tanks, quite a few of them have lots and lots of empty snail shells, and if they don't it's probably because they have the staff that periodically pulls them out (maybe tosses them in the hermit crab tanks for extra homes), that's money they lose. Now I know you may be thinking "but their inability to keep things alive shouldn't cost you more" well that's where you would be wrong it's the cost of doing business, and sometimes in business loss ends up costing the end consumer and even sometimes loss in other areas of the store ends up costing the consumer (i.e. the store tries to make money elsewhere based on sales).
Just looking at liveaquaria I see turbos (currently out of stock) going for $2 each, now try ordering 2 snails and see how much that cost you? I guarantee it'll be more than $3-5 per snail, unless you're also ordering $175 worth of fish and other livestock then you get free shipping (or is it $99? I forget). But I can get a 50 pound bag of potatoes at Costco for $10, how dare Safeway gouge me 99 cents a pound for a single potato! Get where I'm going here?
As to the clean up deals online, not going to deny they are cheap. I don't like them though. First the snails they tend to include are of questionable usage as a "clean up crew", nerites, Florida ceriths and sometimes even margarita snails bleh... (yeah I know LFS sell these too). I have yet to see one of those "really good deal" CUCs that have useful snails like turbos and trochus, I've seen them in packages yeah, but those are not usually the cheap packages. Most of the online vendors I've seen simply collect locally and that's all they do, and Florida while tropical is of mixed usefulness for CUCs. Second literally every CUC I've seen has WAY too many snails for the tank size they advertise, I bought 3 snails for my 40 gallon tank, no more was necessary I wasn't trying to get rid of the algae in a day, because that would lead to dead snails and back to said algae problem.
Now I'm not trying to bust your balls over your statement, but when 1 or 2 snails is really all you need then is paying $8 really that much? Stores are hardly "gouging" you, it's not like they get turbos for 10 cents a piece. But I get you, I feel the "pain" of paying $15 for 4-5 snails when you see how much stuff costs online, but again you're not buying online, and you can't get 4-5 for that price either.