I live in MA and I have bought a car in NH before (lots of people do this). If they have a runner willing to go to NJ, they may do it at a very little cost (DMV fees plus $100)...but some do it at no cost (just dmv fee). It is also depends on the state...how long does it take to do a plate transfer? In MA, you are not allowd to switch plates and drive with out letting your insurance and the rmv know (dealer takes care of all the paper work), so most o fthe time, you cannot buy and take the car home the saem day. However, in NH you are allowed to do it as long as you report it with in 10 days.
Also, since you are buying in a different state, you have to let the dealer know so they can adjust the tax rate (people buying a car in NH will still pay tax for the car...if they do not pay it at the dealer, it will be at the RMV).
If the dealer is unwilling to do the plates, you pretty much have to take a day to go to the rmv and get all your paper work filled out for insurance and the rmv.
Also, when you buy a car in another state, you usually have to get it inspected in your residing state.