What's cheap?

There are filter like adapters that are supposed to give you the fish eye effect. I've never tried them, but have heard they are hit and miss.

You can pick up a used Canon 15mm fish for about $300 - $400. I love my Canon fishy.

There's a Peleng 8mm fishy that alot of people are using. Never used it, but the shots come out pretty well. The color and contrast are good, but some users complain about depth of field on them.

I guess it really depends on the amount of $ you want to spend and the desired effect.