How I would normally respond is:
"Enough to pay the bills"
This way, they'll get that you're unwilling to disclose this information, they'll also know you earn enough to get by.
You should reply with a polite undertone which insinuates that you're saying it in a way that you haven't taken offence to the question, either.
As @Emrakul rightly points out in the comments. Your body language can change this response from a light-hearted and friendly one to a hostile one. So, as recommended a "Smile, grin and shrug" alongside the speech, should suffice.