You are certainly not obligated to answer how much or where it came from. However, I usually answer both questions honestly as I am flattered if anyone likes what I have.
GUEST'S ANSWER: Conner Rohwer, intern for The Oklahoman this summer: If that is his standard answer, it may be more of a habit than his curiousity. Obviously, you don't want to seem rude, but you don't have to share the information with him if you are not comfortable doing so. A simple “as much as I was willing to spend” or “too much” are completely acceptable without really giving away how much you spent. But then again, if you got a good deal on something, why not brag about it?