and others that are cheap are normally [for a reason]
Unlike Avira, which strangely hasn't still been mentioned here (www.avira.com).
AVG and Avast are not so good though.
Anyway, companies which offer a free antivirus are not cheaper but are just using a different (and older) kind of revenue: by offering a free antivirus you are automatically getting an enormous number of daily hits. Since personal users can still buy the advanced editions of the product and companies are forced to do so, in the end even deducing server costs they might earn even more than by just using demo versions (and they can count on a bigger userbase).