I know registration here is (was?) based on weight, is that what's changed, or just the sales tax part? Or, are you talking about paying sales tax on the stated sales value vs. an estimated retail value?
...California has done registration rates based on vehicle value for years. It's a (word for female dog) when they do it that way, lemme tell ya, but sales tax has always been based on how much you claim to have paid for the car.
I can maybe understand why Texas would choose to do it according to book value (not that I necessarily agree with it). Since there's no state income tax here, yet the state budget isn't significantly smaller than many other places, the government has to supplement its income with other use taxes and other fees, such as the rather stiff telephone tax. Doing sales tax on vehicles according to book value rather than stated purchase price makes would thus close the loophole of stating whatever paid value you want to in order to save some tax (c'mon, we've all done it where we paid $2200 for that nice 3-year-old Lexus
)