Cheapest would depend on how many people. We have always preferred to use FL Tours for our transportation. Andy is a wonder guy who will see to it you are taken care of. With FL Tours (or other towncar companies) you can set your own pick up time and not be at the mercy of some one else's "schedule".
Go to www.fltours.com and fill out a request, you will see pricing before you ever have to submit the request for a reservation.
FL Tours is the official transportation company of the DIS and I do not know of any negative feed back ever about them.