Beware of cheap bubble bags.... Black Water Hydro in particular, bought a set and used them once, washed them and went to use them again and the "waterproof" lining was peeling off inside the bags so it would have ended up in the hash. Anyhow about the price, there is so many sizes and numbers of bags i have no idea if that is a good deal. Go to ebay for the largest selection of bags, ask around if any one has bought that particular brand, or type if no brand name and use that as a basis. The one thing about buying them at the hydro shop is you have someone to bitch to after they fuck up. I did buy another set off of ebay that i am very happy with, have run them about 7 times or so and have had no issues, if you want a link to them on e-bay let me know.
I do have to say though that anything smaller than 5 gallons is kind of a waste of time IMO.