Camping in the Key’s isn’t cheap, just cheaper than a room. My kids and I did Key Largo a lot, there is a camp site right on A1A (Caluso SP?) and a nicer one behind the K-mart with a pool but less tent sites. Speaking of tents, you are camping on limestone in the Key’s and most of Florida, tent pegs are useless so bring line to tie your tent to a tree to stop it from blowing away. That happened to us once at Port Canaveral.
There are cheaper tent sites in the state parks BUT you need to break down every morning and then get in line to renew your permit, what time is this, same time you would be out on the morning diveboat! Bahia Honda you camp right on or near the beach, a beach by the way that has made the top beaches list a few times. Site is buggy and the nearby diving is shallow but it’s a short drive to Key West.
Another option is http://www.spreeexpeditions.com/ Frank is cheaper than a shore based dive op and you’ll get in a lot more dives.