Check out Mousesavers.com for a list of discount codes to use with the various car rental agencies. If you have an Entertainment Book, there are several car rental coupons in the back that can usually be used in conjunction with the codes found on the mousesavers site.
Just to give you an idea, I have a minivan reserved with National for 1/6-1/12 (total of 1 week) for $179.74 using the "priceless" code found on Mousesavers along with a $20 off coupon from Entertainment Book.
If you need a smaller vehicle, your price would be even less. An SUV runs about the same.
National is on-site at MCO airport. The price has dropped a couple of times since I intially looked a couple of months ago, most recently earlier this week.
Hope that is helpful. Good luck.
