Aha! I looked up Massachusetts rules:
CIS: Motor Vehicle Excise Information
"An excise at the rate of $25 per one thousand dollars of valuation (effective 1/1/81) is levied on each motor vehicle. . .Figures are the manufacturers’ list prices for vehicles in their year of manufacture. Present market value, price paid, or condition are not considered for excise tax purposes. The excise tax law (M.G.L. c.60A, s.1) establishes its own formula for valuation for state tax purposes whereby only the manufacturer’s list price and the age of the motor vehicle are considered. The formula is as follows:
In the year preceding the model year
(brand new car released before model year) 50%
In the model year 90%
In the second year 60%
In the third year 40%
In the fourth year 25%
In the fifth and succeeding years 10%"
Thus I have a much heftier bill to look forward to next year 
