The problem answering your question is that the rules and rates for registration, insurance and taxation varies from state to state. The big issue here is to try to avoid paying taxes twice, once in the state where you buy it, and once as "use tax" in the state where you register it. Also expect Texas to be very slow about issuing permanent Title paperwork, I live in Louisiana (Title is proof of ownership, registration is paying for your license plate), I live in Louisiana about 20 miles from Texas, my wife has bought her last 2 cars as used cars in Texas, both times from dealerships and both times the 60 (90?) day temporary paper plates were within a week of expiring, before the Texas paperwork arrived that would allow her to get permanent Louisiana plates.
I bought my current coach in Florida, and drove it home to Lousiaina, thankfully I was able to get a Louisiana temporary plate from my bank who financed the coach to drive it home instead of dealing with the Florida DMV, as the standard Florida procedure is to pay 8% tax to get a temporary plate, drive to the home state, pay tax again, then one has to file paperwork with Florida to refund the difference if the Florida, assuming they have a receptacle tax agreement, which at the time they did have with more states, though I think North Carolina was an exception. So buyers from NC buying in FL would be stuck paying tax twice.
As to the rest of it I have personally seen sales tax on vehicles range from 3%-9% depending on the state, my state charges 9% of the book value of a vehicle when it is brought into the state and registered here. A number of years ago I moved back to Louisiana after living in Alabama for 6 years, while in Alabama I bought a car, put 90,000 miles on it, and paid more in use taxes to Louisiana when I moved back than I had paid to Alabama when the car was brand new. Note with my state the 9% is on book value, not actual sales price, so even if you get a great price buying out of state you still pay on the higher book value. Of course by contrast registration in Louisiana was $25 per year and it was over $250 in Alabama. That reminds me I need to renew the registration on my motorhome next month ($50 for 2 years yay ) thanks to it being over 12 years old. I renewed it for my 8 year old car last month at I think $65 per year. I recently read on another site where some new expensive diesel pusher owners are paying $3,500+ per year for registration in California and New York.