I haven't known many who didn't ask in some way, at some time. If they don't ask directly, they still find ways to try to get me to tell them.
I don't mind. It's a natural question, and an important one for many of them. Even when it's just a nosy old fart, I don't care. Money is how most value the worth of anything, and I don't mind this, either. It's largely true, especially in the business world, and the monet you try to sell your writing, you're in business.
It may be that writers don't notice, but people ask this same question about every activity and occupation. I would, too, if I had any thoughts at all about entering that occupation.
This doesn't mean we have to tell the, and there's nothing wrong with a polite, "I don't tell anyone that", or even a nice, "It's really none of your business".
My answer changes depending on who is asking. I rarely say exactly, but if it's someone who isn't making much money, who has that desperate look, who prays every day for a chance, I give them a pretty fair estimate, and talk to them about what it takes to be a writer.
If it's someone who just wants to make certain that a writer can't possibly make as much as he does, I wait until he swallows a couple of antacids, and the play along. I'll often say something like, "Oh, I don't take money for my writing. I think big business and capitalism are the root of all evil, and those overpaid thieves who get rich of the backs of the poor should all be shot because they're just minions of the devil. Whatever money I do make is split between Communists of America, and Kill The Rich Demons, Inc. Not a penny of the filthy stuff reaches my bank account."
Really, I can't believe more of you don't have people ask. It really does seem that everyone I meet asks sooner or later. They not only want to know how much I make per year, but how much I made off every little article, short story, or recipe I've sold.
I really do understand why people ask, and it very rarely bothers me. It's not a national secret, and it's not like they're asking something with an answer that would embarrass me. I know what it's like to be dirt poor, and what it's like working your butt off on a dead end job. Far more often than not, something like this is behind the question.