Outside of hybrids and electrics, less expensive cars generally have better gas mileage than more expensive cars.
This kind of technical-defining, peripheral tweaking, and loophole making is likely to result in a bad law.
Worse, depending on the defining and tweaking, I'm not sure this is true (source?). Which kind of "expensive"?
1) Sticker price at time of purchase
2) Value vs other cars sold in the same year
3) Value right now.
You should answer this, but at the same time, note that you're doing it the hard way from a public-policy standpoint. The dollar value of a car is still several levels removed from things that are undesirable from a public-policy standpoint and will result in all kinds of silliness (are aftermarket lightweight alloy wheels good because they save fuel and roadwear or bad because show-off are bad? How about fat tires? Or Ford charging more for eco-boost V4s and less for bad-mileage V6s?)
We've learned that road wear is proportional to the axle weight to the 4th power. Road wear is expensive. So tax gross vehicle weight to recover the state's cost of road repair. (of course since heavy cars get worse mileage, a gas tax works too).
Actually driving causes road wear, so a VMTs, Gas tax, and tolls a seem good. A gas tax is the easiest, cheapest and least-intrusive to collect. Side note: Leno's car collection, stowed in barns, is costless to society: no driving, no parking, yeah! So maybe rich people should be actually encouraged to take drive-able vehicles off the road.
Driving at *rush hour* causes congestion and triggers the need for more roads. Congestion charges (variable tolls) get at this directly--and rich people are more likely to pay them!
Parking cars (and cheap parking) clogs roads. Performance parking in Boston (as spots get scare, prices go up...DC is trying it) would work like a congestion charge. That, plus a tax on garage parking (raising the one we already have) could be devoted to MBTA. (if rich people drive every day, they'd pay it)
A Tolled CBD congestion perimeter is less acceptable to downtown merchants than performance parking, but has been shown to be very effective in de-congesting the core and promoting (and funding) transit.
In all your proposals to tax expensive cars seems just a "class warfare" thing and really doesn't advance any public policy objectives better than existing (or available alternative) fees & taxes. I just don't see that high-value cars are "bad" in any way that you'd want to make discouraging them a priority. A lot of American jobs are devoted to making those big, expensive cars (including BMW, Mercedes, and other "foreign" marques).