There is no shore diving in the Keys. Why not consider boats in the Keys? You can get some very cheap hotel rates and there seems to always be a dive op within a mile. Go to Expedia and look for "discount hotels in Key Largo". You can find a decent room with a pool for about $100 a night (Marina Del Mar for instance). If you really want to go "economy" you can actually rent a room at the Sunset Inn in Islamorada for $50 a night. Mind you, NO amentieis, just a room, great for me and a buddy but I don't know if my wife would stay there. She likes the pool and other stuff.
I could actually book a trip using Sunset Inn and Rainbow Reef divers, for five days, doing 10 dives with a DM included on every dive for $575. Again, no frills. Rainbow Reef has a package of 10 dives (with DM) for $325. And Sunset Inn would cost $250.
That's just an example. If you can make a decision between "what you want" and "what you need", diving in the Keys is fas less expensive than diving the east coast of Florida. As you said yourself, $120 for a room far off the beach. Use this internet tool, you'll be amazed at what you can find.