I'm not very graceful with my comebacks, I tend to come off verbally abusive when confronted with the sort of arrogant reactions that you've been getting.
Maybe you can just be like "I didn't really feel like spending 'new car' money on something I wouldn't enjoy driving", or "If I was worried about gas prices I wouldn't really be paying 30 grand for a car now would I?"
or better yet "i'm actually banking on prices coming down since I know you and most others won't be using as much of it in your civics"
and lastly "yea you're right, it totally sucks having a big, powerful, capable, safe vehicle, totally not worth the marginal difference in gas money"