Suppose two people make the same amount of money. One chooses to buy a cheaper car and pays higher rent for a nicer apt. The other, the opposite. Under your pricing scheme, you're unjustifiably charging two persons of equal means different rates.
You can adjust this many ways. One person gets a cheaper car but spends more on luxury vacations or invests more aggressively or spends large sums eating at nice restaurants often or buys expensive clothes or or or etc.
Edit: Lastly, why should we be charging expensive cars more? A BMW M4 is, by all measures, much less irritating to have to share the road with than a large SUV or Ford F-250 (god forbid it's also lifted). Tolls should scale with vehicle size and weight and when vehicles have poor fuel efficiency, not the sticker price.