I can't imagine this would be cheap and I can't imagine the rental location you are getting the car from being happy to let you use one of their cars (in my experience, just doing a one way rental to a location 30 minutes away is difficult because the original location says it's hard to get the car back and then they have a car less in their inventory).
But I'm curious to know what you find out!
FWIW, I just tried from Costco Rentals and for a one week rental from July 6 to July 13th with pickup at YYZ and drop off at MCO, only AVIS offered it. It's $662 for an intermediate, $691 for a full-size and $11,650 (yes almost $12k !!!) for an intermediate SUV !!! (Only those 3 vehicle categories were available). To put it in perspective, MCO-MCO with AVIS is $234USD / $285USD (Alamo is $265USD) / $277USD (Alamo is $265USD) for the same 3 classes so there is a premium but it's not actually as much as I thought.