If you are coming from MCO, get the Visitor Toll Pass at the airport. You rent it for $10 but that is refunded when you return it so there is no cost for the transponder - you only pay for the tolls. A friend of mine used it and said it was real easy. And much cheaper than buying the rental car toll pass!!!!
Visitor Toll Pass - Welcome
If you are going to Brownwood, you would exit at Wildwood 301 (or if you wanted to see the new area under construction, you would exit at 296 route 470) and take SR 44 to Brownwood - about a mile.
I did the numbers and it is $6.15 each way - if I did it right.
SunPass Toll Calculator