Here's the thing when it comes to snails and clean up crew... You're coming from a freshwater world where nuisance plants are pretty much a couple varieties of algae that are really only eaten by a couple species of algae-eating fish (ottos, plecs, siamese, etc). In the marine "nuisance" world you have green algae, brown algae, hair algae, brown slime, red slime, green slime, and the list goes on. There is a BOUNTY of differing types of nuisance growth in a marine tank and there is no single catch-all for helping you with all of it. Hermits do great on diatomaceous algae, some green algaes, and uneaten food. They won't touch hair algae, or various versions of slime algae (usually cyanobacteria), nor will they help clean your glass. Some snails eat hair algae, some dont. Some snails eat cyanobacteria, some dont. And so-on and so forth.
The general rule of thumb for CC (cleanup crew) is 1 per gallon. In my 45 for example I have the following:
10 blue legs
4 red legs
6 mexican turbo snails
2 nassarius snails
8 cerith snails
1 emerald crab
6 astrea snails
2 strange un-IDed snails
1 lettuce nudibranch
See the variety? They each do a specific job, and without one of the guys on that list, I might be hard-pressed to stay ahead of some nuisance critters. Of note, the ONLY critter I've found amongst that list that eats cyanobacteria are cerith snails. Variety is the spice of life when it comes to CC
Prices might not compare on the other side of the "pond" though 
Ive seen crabs caught in the tide pools escape before, and they met their untimely end. 
