Unless you find a freebie, your cheapest solution and a reasonably safe one is at a mooring in Boot Key Harbor, Marathon. Theoretically moorings are for live aboards- transients and full time and not as a place to store the boat, but lots and lots of people do it. Just don't tell the City Marina that is your intention.
Sometimes, particularly when the wind is out of the north for a while and the Bahamian crossing is backed up, the moorings fill up. But if you can wait a day or so they will call you when a space opens up.
Slips in the keys are always expensive.