I think everyone has the right idea with the comparisons. I remember someone else relating a story on this forum (or maybe MTBR.com) where he was talking to some hunters about his bike. They were amazed at how much he spent on his bike - about $1000. He asked them how much they spent on their hunting rifles and all their hunting clothes. It was about the same amount, a little more. Then he asked how much out of the year they really got to use their hunting equipment. About a month. He told them he rides his bike nearly all year long. They promptly shutup.

Do your parents have any expensive hobbies that you can make the comparison to? Play golf or something like that? If they do I'd try to make an analogy like that in addition to the car one. The bottom line is you get what you pay for...