I buy them for 1 dozen for like $1.10 or something weird like that. They do a great job of cleaning, but you should be aware of two things:
1. If you feed your fish bloodworms or anything like that, the ghost shrimp are very fast, and are happy to grab the bloodworms away from the fish
2. You need to have a lid on the tank, because they like to do this weird thing where they skim the top of the water upside down, and the can flip themselves out of the water pretty easily.