I've never been asked to buy anything from avg, avast, lavasoft, spybots&d, xvid, gimp, vlc, tcmp, etc.. etc...
I do however pay for my antivirus (not the above mentioned), and have had no problems with it whatsoever considering spyware, virus', adware etc. Maybe not everyone can afford to pay, that is true, but it also doesn't automatically make those programs any less than one you pay for. If you look back on my previous post, I had a paid subscription to Norton's internet security thing, and it did a crappy job compared to the free ones. Not to mention, a lot of paid software do not have trial versions for you to test out before buying.
As far as I can see, nobody said you couldn't pay for an antivirus, everyone has recommended the free ones because they do work. Even though I pay for an all in one, I still use a few of the others to double check.