How much tax do we pay.
Any purchase from any dealer will require sales tax; state, occasionally county and often city. In most places this will be from 5 to 9% of the sales price. There may be a state or two with no sales tax, but I doubt it. There are a few states which tack on a luxury tax, but I don't know which ones. Any taxes which are required will be required whether you are a resident of the state, the country, the planet, the solar system, the galaxy or even another dimension
If you buy it from a private party, you may not have to pay tax, but in many states you do have to pay the tax when you register it. Which brings up another cost.
Once you own it, you have to register it. This varies from a flat rate in some states (between $50 and $150 usually) or a 'tax' based on some factor such as size or weight, or worst of all, value. I don't know how you would register the vehicle without a permanent address; perhaps the transient friendly states such as South Dakota, Florida or Texas might make it easier.