I wish I could help more, cabrits, but for the major car rental agencies, this is pretty reasonable. On a per week basis, at about $250 per week, the nominal figure is around $1000 per month.

Having said that, I have gotten some terrific deals from Enterprise. They do have weekly and monthly rates at a lot of locations. Give them a call. One disadvantage is, they may not be at your airport, but at least in my area at least, they wsill deliver your car to you AT the airport. And drop you off. Not too bad a deal!