I got a gorgeous rock from my finace (due largely in part to his parents providing the center stone) and let me tell you - a nice rock is NOT worth the trade for a douchebag husband!
If she loves you it doesn't matter if the rock is huge, fake, or a semi-precious stone. You can get HELLA nice lab created stones cheap that are good size and look real. (I know, I buy myself enough
)
There are also really beautiful rings at stores like Zales and Kay's that are under 3k.
Don't trade the car for a ring at all - I agree that you'd regret it and that would ruin the whole purpose of proposing.
There may be some women that need huge exepnsive rocks - but frankly, I think your love for her (and hers for you) is far more important then a stone. Get her something you see that reminds you of her - her style, her sparkle
and who cares if it only cost a few hundred vs a few thousand.
(again I pimp the lab created stones - they are just good as the real sht - in fact, better b/c they are PERFECT and they have so many more choices available...)