Depends on the source. Locally, if you pick up fiddlers at a wholesaler, they sell for about a dollar. If you have them delivered it costs more. So, 4 may be a little high, but not to bad considering the overhead that comes with a live animal to the public business.
I guess if your only buying one as a pet, my sister keeps one in a gallon tank on her nightstand, it would be o.k. to par 4 dollar's a crab... but when you need 4-10 or even more a month you have to either go strait to the source or find someone that doesn't have such a high overhead.