You have THREE choices then. The free route, the faster route or a combo of both.
If you take the Turnpike all the way it will cost around 20 bucks to get to Miami but you can do it in as short a time as 2.5 hours. You would have to drive about 80-85 mph to get that done. If you drive around the speed limit 3.5 hours can easily be done, short of rain or accidents on the highway.
The free choice is to grab highway 27 which is about 15 miles west of Universal via I-4. On 27 however you will get a stop light every 15 miles or so and there are a ton of 18 wheelers that use that highway.
The 3rd choice is to take the Turnpike about 117 miles south from the I-4 entrance and then get off of the Turnpike at the Fort Pierce (SR70) exit #152 and head east about 1/4 a mile once you exit and then grab I-95 South to Miami. It will cost you about 10 bucks to do this. This is the route I take because I usually have business in the east side of Miami which is where the stadium is as well. If you have to travel to the west end of Miami it is best to either take the Turnpike all the way or to grab 27. If you are going to get to where you are going before 8 AM on the weekend I recomend highway 27. It is a safe road up until about 9 AM or so. It's a great road to drive from around 8 PM to 8 AM. And if you plan to get to the stadium before 8 AM you can make that drive in around 3 hours on HW 27.