If you book through Expedia, Hertz car hire are offering a special rate if you hire a vehicle for between 5 and 27 days. The offer is available at Orlando International airport (offsite) and Sanford (at airport). The longer you hire the better the deal is.
I've just booked a 4 wheel drive 7 seater SUV for 3 weeks in October, and it's cost £266 (same price as 2 weeks). This would typically cost £400 more with other suppliers.
There doesn't seem to be much cost difference between hiring a car for a week, or 27 days!
This price includes all relevant insurance for one driver.
Don't forget Quidco which gives 6.9% cashback for carhire through Expedia.