Steve,
While I've not registered my GT40 here yet, I have had customers and friends register their kit and component cars here and the sales tax is always based on the purchase price of the kit and driveline. Inspectors know the values of these cars and they talk to each other to compare notes. Some are better / nicer than others.. Most people from the Twin Cities I know actually go through the DMV in Faribault (only 15 miles from me) as the hassle factor is really low and there are rarely lines to wait in longer than 15 mins..
This is truly the land of 10 thousand taxes and I just don't know any way around it unless you register in another state and the "move" here....