It really will depend on which agency that you rent from, they have very different programs.
Alamo/National/
Enterprise has the best system IMO. They charge $3.95/day plus the cost of the tolls... ONLY ON THE DAYS THAT YOU ACTUALLY INCUR TOLLS
Budget/
Avis/Payless also charge $3.95/day plus tolls, however they charge it for everyday of the rental...not just the days that you incur tolls.
Hertz and Firefly charge $4.95/day plus tolls for each day of your rental, not just the days you incur tolls.
Dollar and Thrifty charge an all-inclusive rate of $10.49/day x the total number of days of the rental (includes admin fee and cost of tolls)
The $20 Sunpass transponder can be a good option if you will be incurring tolls for 4 or 5 days (or more) since even with Alamo's system you will basically be paying the $4/day x 5 days = $20 which is the cost of the transponder. The plus would be that if you plan a return trip to Florida anytime in the future, you will already have the transponder and not have to pay anymore fees (except the tolls of course).
Another plus if you own the transponder is that you get a small discount on the toll (10-15%).
We have started visiting Florida 2 or 3 times/year since 2010 (starting to spend the kids inheritance

) and we quite often end up in, or fly into the Miami area so we purchased a Sunpass Transponder a few years ago. It's been great and can be a time saver, I wouldn't leave home without it now.