As a general rule, *nix hasn't been a big virus target. Obviously the more popular coding points towards MS based systems due to the higher number of end-users. Until more recently, few known and prevalent viruses for linux/unix even existed. And most of the new ones have just used exploits in linux (many just worms and whatnot) for perpetuating themselves. These are often just repaired with security patches by the software maker for the software that was exploited. If you want to stay on top of the security issues for linux, go to
http://www.linuxsecurity.com/advisories/index.html and choose your favorite flavor. This may not be bleeding edge for security warnings, but it's pretty darn close.
I have been on and off the linux scene for a couple years and have yet to actually see or hear of any of my peers get any type of virus in linux. Yes, they do exist and yes, there is probably a risk in not having some type of protection, albeit minimal.
The anti-virus software packages for linux are many times for scanning mail servers/clients for incoming mail w/ viruses and scanning windows/dos files for known viruses that affect those MS based systems.
IMHO, I would say that you probably needn't worry about getting any AV software let alone actually paying for it.
If, though, you are still interested in AV software, check out (what appears to be) free av software for linux from BitDefender at:
http://www.bitdefender.com/
I saw that they have a free linux package of some sort but haven't checked it out in depth myself.
Login 
Linux and Unix 
Search 
Facebook
Twitter
TechGuy.tv
TSG Mobile