To reduce the cost of a 3 month stay in Florida I would suggest that you spend some time researching the different campgrounds and parks rather than throw up your hands in disgust. There are plenty of lower cost parks, but you have to look for them. Try RVparkreview.com as a tool for finding parks in an area you're interesting in staying.
http://www.rvparkreviews.com/If you want to stay in the high demand areas along the coasts you're likely to pay plenty. That's the old supply & demand issue. If you take a look at US27 winding it's way down through the middle of the state you will find some acceptable prices. From Ocala down through Winter Haven to Sebring there are several reasonably prices parks, but again, you will have to do some research to find them. The lower cost parks may not have all the amenities of the higher priced parks though.
In regard to looking for a place to camp near Homestead for a reasonable rate you might want to look at state parks along US41 in the Everglades. It may still be a bit of a drive to the track, but the prices may be better. Anywhere around a Nascar track during a race weekend will be expensive.
For checking out state parks check here:
http://floridastateparks.reserveamerica.com/welcome.do?topTabIndex=ReservationHome