In Massachusetts you must be 21 to rent a car, and rates may be higher if you're under 25. When picking up a car, non-U.S. residents will need a reservation voucher (for prepaid reservations made in the traveler's home country), a passport, a driver's license, and a travel policy that covers each driver.
Base rates in Boston begin at $25-$40 a day and $170-$250 a week for an economy car with air-conditioning, automatic transmission, and unlimited mileage; rates tend to be slightly lower out of Providence and slightly higher out of Hyannis, but much depends on availability, which changes regularly. Keep in mind that taxes and a variety of surcharges typically add 30% to 40% to your total bill.
You can book car rentals on Martha's Vineyard through the Woods Hole ferry terminal free phone. A handful of local agencies, listed below, have rental desks at the airport. The cost is $90-$150 per day for a sedan. Renting a four-wheel-drive vehicle costs around $160 per day (seasonal prices fluctuate widely).
If you're determined to rent a car while on Nantucket, be sure to book early. Expect to spend about $85 a day during high season.