Personally, I've never asked anyone that.
I don't ask my doctor, or my garbage collector, or my best friend.
I may have wondered, heck, I may have even tried to look up a ballpark figure, but I've never ASKED anyone that question.
As a writer, I get asked that ALL THE TIME. I've been asked it twice already this month.
And I know I'm not alone in the experience because other writers complain about it bitterly too.
I won't speak for others, but on my personal list of IRRITATING THINGS WRITERS HEAR it falls at number 3, behind: "I have an idea for a book, but I just don't have the time to write it" AND "Are you published?"
I did an informal survey about it on Facebook this weekend. (If you're not a FB friend, you should consider becoming one...I'm right here) I was surprised to learn that some people in other fields have had to deal with same question, but almost all writers had.
Is it a secret?
Is it anybody's business?
I did however like someone's suggestion of just saying, "not enough".
Or maybe I should just bore them to death with the WHOLE answer which includes really boring stuff like production costs, self-employment taxes, being self-insured, and significant work hours.
Tell me Killer Friends: Have you asked that question? Have you been asked? How do you feel about it?