Alamo/National are on-property at the Dolphin and the Car Care Center near MK.
There are also other companies at some DTD Hotels, but I find it the easiest to take a bus to the Dolphin (or boat from MGM and/or Epcot) to pick up a rental. Also I find National/Alamo cheapest with coupons/codes.
Can't answer your question about taxi fare, but I bet renting a car for a day might be equal if not cheaper in price!!
Good luck!!!
