If you are or ever going to buy a car in Va. read this

Unless their website hasn't been updated, it says they need to use NADA on a car 5 years or newer. 5 years and older the tax is 3% of purchase price OR 35 dollars which ever is greater. The purchase price on the title IS considered the "bill of sale".

You can read it here.

http://www.dmv.state.va.us/webdoc/citizen/vehicles/sut.asp

I know it's too late now, and you will have to follow through with your other recourse but it sounds to me as if the person you dealt with didn't know what they were doing.