We have done this twice, most recently earlier this month after back-to-backs on the Magic. We flew into MIA on American and out of MCO on Southwest, so only did a one-day rental from MIA to WDW. We happened to use National bcause I had a free day coming, so we dropped off the rental car at the Dolphin the morning after our arrival at the BoardWalk. The drive from the airport car rental center was about 3.5 hours, including a lunch stop.
I actually prefer to take I-95 to 528 to 417 rather than the FL Turnpike. Toll cost was a total of $10.45: $5.45 to National for the Miami area toll(s) using toll-by-plate and including National's $3.95 toll service fee for the toll-by-plate service, two $1 tolls on 528 and two $1.50 tolls on 417. The tolls on 528 and 417 were paid using the cash lanes. I have the Illinois version of E-ZPass (called IPass), so not usable in FL. I have not compared the tolls on the FL Turnpike, but I believe the Miami area tolls to get to the Turnpike is probably the same as the tolls to get to I-95. Add the Turnpike tolls to this to get the total.