This is similar to how it works here. We do not have titles per se, however we now use a transfer of ownership document from seller. Tax is due on the book valuation or the purchase price on the document, whichever is higher.
I just went through a couple of my project cars that I after many years am getting moving on, and when I went to register them they wanted to tax them on values of completed cars which was around 30,000 each, regardless of what my bill of sales from the mid 80s indicated. I instead paid a certified appraiser to provide valuation as they sit today, and since all the parts are off, and they are simply shells, the value was cheap. Just glad I didn't get them done or close to done before paying the taxes.
