I can put you in touch with a guy in the U.K. who exports to the U.S. for a living if you'd like to get some specifics. In addition to the shipping charges there's a fair amount of paperwork to sort out on each end. On this side of the Atlantic there's also a broker's bond or fee that has to be paid. In the end, the amount of hurdles and the total expense don't often justify the effort.
However, IF the '73 you've been offered is in sound shape (mainly the body), this does sound like a car that would be cost effective to import. Keep in mind that bodywork costs a mint regardless of whether it's a Mini or a Porsche. Make sure the car is nearly rust-free with very sound sills, footwells, and A-panels before you consider importing it.