I agree that "nunya" is a good response, but if you want to be polite then a non-commital "not much for an investment piece" or "an arm & a leg" are sufficient. People who are rude enough to ask how much something cost when they just want to "score points" re: their perceived frugality vs yours won't take subtle hints anyway. They are probably the same people who think it is ok to touch the stomachs of pregnant women and make comments re: how many children someone has.