A single workstation license for f-prot for commercial use is only $29. If you can't afford $0.08c a day to protect your users ... well ... you probably aren't a commercial site and might as well use the freebie "home use" version!
Having said that, clamav is hands down a better product, and its free. So I certainly wouldn't object if Jafo happened to mod his mod to use it.
-- hugh