The Keys was our first destination after we got certified in 2010 and it was expensive THEN. It's only gotten worse - and so much worse - over the past few years. We've been back several times, but we invariably ask ourselves if it's really worth it. The diving can be great, but often isn't. There are some great restaurants, but they're expensive too. It's not as bad as Hawai'i but it's getting close. The one big difference is airfare (if you live in the US).
AND ... AirBNBs seem to price themselves out of most markets when I start looking at them - with cleaning fees that are often equal to a night's stay, and other fees/taxes. If you can find someone who owns a place there and rents directly you might find a good deal. Maybe join a Florida Keys Facebook page? I know there are a couple of good Cozumel groups and people will sometimes post about their rentals so you can avoid going through a middleman.