Since your looking at a major expense, I suggest doing some research before you make a final decision.
The "major" rental car agents don't normally deal with long term rentals. Most do have monthly rates, but they are unpublished so you have to call and get them. I'd even consider calling the local station and speaking with the manager, as he/she might have negoiating authority that a reservation clerk would not have.
You might also consider callling "off-airport" rental agents, like Enterprise for pricing (BTW, Enterprise will quote long term rates on their web site - I tried it). In addition, consider alternatives like rental offices of car dealerships in the area. If you do use these, remember they only operate during standard business hours (not 24hrs like an airport location)and won't provide an airport shuttle.
There's more to consider. Ask about restrictions on where the car can be driven. Some agents may not allow the car outside of certain states or in Canada. Also, they agent may require the car be inspected every thirty days. You should also check with your insurance carrier to verify your coverage includes long term rentals.
Finally, I found an old (1998) Boston Globe article that talks about Long Term Car Rentals. It's worth a read:
http://search.boston.com/travel/cons...998/0401.shtml
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Michael Steinberg
Editor
BizTrip
http://www.biztrip.com