You can get a "SunPass" transponder at Walgreens for $19.99 and put it on your windshield or get a barcode sticker for $4.99. Precharge your account with $25 and set automatic recharge to trigger at $10 charge your card for another $25. That's the best way. If you let them bill your plate, they tack on a service fee.
WELCOME TO SUNPASS - Florida's Prepaid Toll ProgramCall them on the phone to set up your account - it can't be done online (ask me how I know).
I was just down on Key Largo a week ago - I was surprised to see gas prices at $2.39 much the same as in Homestead/Florida City. Camping fees at private parks are upwards of $1200 depending how close you are to Key West. While that price is quite high for my taste, the lots are often so small you may not be able to open your door let alone put out your awning.
Another option is E-Pass (which I wish I had known about before I bought the SunPass transponder)
E-PASS vs. SunPass: Which is better for drivers on Florida toll roads?E-PASS stickers are free.
E-PASS portable transponders are $9.95.
E-PASS Xtra, which costs $18.50, works in 18 states; SunPass does not have a similar, upgraded version.
Get E-Pass - Which E-PASS is right for you?As another option, come south on I-75 and get off at the Collier Ave exit in Naples (the last exit on I-75 before it becomes a toll road). Take it south to US-41 (The Tamiami Trail) which parallels I-75 across southern Florida. Turn south on Florida 997 (Krome Ave) which runs into US-1 just south of Florida City. No toll roads on this route and there's several attractions you may want to stop (and federal campgrounds). Try an airboat ride with the Miccosukee Indians.
Tamiami Trail: Scenic drive exploring Everglades