Sunpass transponder is $20, but the CFX Portable E-Pass transponder (not EZ-pass) is only $9.95 and appears to function throughout Florida:
https://www.cfxway.com/e-pass/get-e-pass/. We have used ours for the Orlando-area toll roads, the toll roads along the Gulf coast, and the toll bridge from St. Petersburg to St. Pete Beach without any problems. I ordered online and paid a total of $10.70 for the transponder, including tax, shipped to my home in about a week.
Apparently, the CFX web site is not as slick as the Sunpass site for entering rental car information, but the Portable E-Pass does the job for us at about half the initial investment. I also have EZ-passes in our personal cars, but they would only work on the CFX portion of the FL toll system and I wouldn’t want to deal with the much less convenient web site of the Illinois toll system to enter rental car information.