Lots of cheaper options, down to and including duct taping a piece of dirty window glass to the front of the lens.
The best cheap option is no filter at all.
I got a Tiffen UV Protector with my 15-85, cost me something like €20.
After a while I did a test comparing IQ with it on and off, the difference was so horrible that it's now gathering dust.
Cheaper would have been no filter...