I flew in to KBED a couple years ago and used Signature, it was very expensive though I don't remember how much. Now I've moved to Boston and fly out of the same airfield with East Coast Aero Club (but they're not an FBO and don't offer parking AFAIK)
Jet Aviation is apparently a bit cheaper than Signature.
If you don't want to pay for a rental car there is a public bus, #76 (weekdays) or #62 (Saturday), which rns about every hour - check mbta.com. It will take you to Alewife subway station.