I wouldn't give this a serious answer unless I were really close to the person - only because I don't see how the question could be asked without having some reason (ulterior motive) behind it.
I guess a polite way to couch it is saying something like "I make ends meet". If they can't get the hint on that one and they insist on knowing (and you aren't willing to share), you might have to be a bit more firm with your response:
"I'm sorry, but I am not very comfortable sharing that information".
Or you could counter it with an inquiry:
"I would feel more comfortable answering if I could ask you, why do you want to know?"