This is one of the most asinine questions you can ask another man (if you're a man). I'm not sure how this works with women, but I suppose it's about the same.
If someone asks you this, the obvious answer is, "More than you, obviously." But you must just quietly say to yourself, "Purse my lips," and try to be tactful in the face of this abject rudeness.
I had a good friend whom I've know for several years ask me this last year, and I have not really cared to see him since. It changed everything between us, and we've said stuff to each other that would curl youngsters' ears to the point of deafness for life. This, however, was inexcusable. I could tell he wished he hadn't said it, and I actually told him, which I wished I hadn't done.
I had a another friend who was a go-getter and wound up in the advertising business (see Johnny and Mary if you want to know more about what that will do to you). He said that you should be earning your age. For instance, if you're 30, you should be earning at least $30,000 a year. That was back in the early 1980's. I wonder what the benchmark for young working folks is now?