Mears is about the best bet price wise for a single person. You might be able to get two one-way car rentals on either end of your trip for somewhere in between the cost of Mears and a towncar.
One trip we took, we only needed transportation for getting to WDW, I found a National mid-size for $32.50 (fees included) for 24 hour. I picked it up at the airport and dropped it off at the Dolphin. You just need to check out the National/Alamo website to see what they have. There also is an Avis or Hertz office at one of the Downtown Disney hotels on Hotel Plaza Boulevard.