[69hugger]

My opinion is it's rude.
I think it is less impolite ask a person what "something like that is worth", because it puts them less on the spot. They can choose to tell you what they paid, give you a range of similar cars, or they can give a vague response like "it really depends on many factors, such as suspension upgrades, etc".
I get asked pretty regularly what I paid for my cars. It really puts me on the spot because I don't want to come off like an a-hole, but I don't know the person from Adam, and it's none of their business. I usually say something like "more than my wife thinks".
I think it holds true for houses, boats, motorcycles, etc.

Just my $.02...