I see you are in Virginia . I went to dmv web site, it states they use nada or sale price if bought from a non dealer. Was sales tax paid on it when you bought it? If so you are owed a refund. Here in Texas it is based on 80% of car value using nada. If you pay more than nada and tell them, they base it on the highest price. Just check dmv wed site it"s on there. Registration is based on weight. Appears like they were charging you sales tax on the original purchase. Dealer should have paid that when u purchased. Some states collect personal property when u reg but tax would be based on nada . Check it out!