I wouldn't rely on a $3/month web host for my one and only copy, that's my point.
But you probably wouldn't use $3 hosting to begin with. The kind of customers who are attracted to this are exactly the kind of people who don't have backups and expect great service for $3. (Because, after all, $3 is a million times more expensive than Google, and Google is pretty reliable...)
Forget about how much you think it costs to host your backups. Think about how much you backups mean to you. What would happen if you didn't have access to your backups?