I've flown Cape Air from Boston to Martha's Vineyard - they're great! True, their current frequent flyer program is the 10 coupon flat rate discount that
fastflyer mentions above, but all my flights were on time with friendly service. They fly 9-seater Cessnas with one pilot and they seat you by weight - I got to sit next to the pilot each time.
They will also fly one pet per flight in the cabin for $10 each way, which is nice (FF's pets fly free). More info on Cape Air at:
http://www.flycapeair.com
This is only a 25 minute flight. I've been to Provincetown by car and it wasn't fun (well, the weekend part of the trip, at least - a massive traffic jam). I priced flights on itn.net for mid-June and came up with about $199 round trip. I wouldn't hesitate paying that in a second and renting a car once in Provincetown. I saw some promotions on another site that were as low as $99 BOS-PVC round trip and one fare with a major carrier from New York City through Boston to Provincetown for $198 round trip.
http://www.provincetown.com/plan/get_here/get_here_air
Note: If you plan just to hang out in Provincetown, you really don't need a car (as mentioned in a previous post). My favorite beaches were about 10 miles away in Truro (ocean side), though, and the rest of the Cape is beautiful to visit. If I did it again, I would find a house to rent in Truro and drive into Provincetown for restaurants and nightlife.