I don't see why you'd need to. Say MathOverflow put a nominal $1 fee before you can post questions just as a form of quality filter. Would you then not put a link to that site because it's not completely free?
You should link to think that you find useful, and awesome, and underrated. We don't need advertisements for four function calculators or Mathematica, but perhaps you know of a very cool group of passionate people that deserves more exposure, or an awesome tool that you feel might come in handy to the audience of the site. The price tag then doesn't really matter.
I've posted an ad about LyX, for example, on the presumption that writing about maths is what most regulars do here, and doing extensively so with the most popular writing software, Microsft Word, is an absolute disgrace. Even if LyX wasn't free and libre, I feel that'd still be a very appropriate ad for the site.
At the end of the day, the community ads aren't free to post anyway. If you want your own activity advertised on the site, you need 5 reputation (as a moderator on another site, I can tell you this barrier is not as insignificant as it sounds: those first 5 points are the hardest earned ones) and 6 upvotes from other users not associated with your product.
If you feel somebody is being excessively greedy, you can just vote their ad down. It's still not a good idea to ban community advertisements tout court for anything that comes with a price tag.