When I worked for Enterprise in the Boston area we only charged $15/day for under 25. I'm sure it varies by location, but if you must have a car I would check into Enterprise. Typically not on airport, but they usually offer a shuttle.
However, if money is a concern, I agree with Kermit. Check Tiffany Town Car (I believe they're an advertiser on this web site). Last I checked they were charging something like $75 RT to most WDW resorts ($80 to a couple of MK resorts). You can feel like you're vacationing in the lap of luxury AND save money over a rental car. My understanding is the driver meets you at the baggage claim, helps you with your luggage, takes you directly to your resort, then picks you up in time to make your return flight. WDW transportation will get you everywhere you need to go on property, as long as you allow the necessary time.
For less money you can also go with Mears, but we weren't impressed when we used them. We were left waiting on the curb at the airport for quite a while because no one else was going to the WL and the dispatcher seemed to be waiting for others to make it "worth their while." When I expressed my impatience he put us on a shuttle, but we had to stop at three or four other resorts before being dropped off at the WL. Nice tour, but I wanted to start my vacation, not ride on a shuttle.