Although this is top of the season, there are still places that are reasonable. Our favorite is the Courtyard Faro Blanco Resort in Marathon. We have been going a couple of times a year for years now and love it. This time of year will run you about $300 a night for a week stay. The best part of this location is that it is an a little under an hour drive to Key West so you can go and spend a day in Key West and get out before the mobs of kids from Miami arrive.
There is also a great little dive called No Name Pub in Big Pine Key on the south side of the 7 mile bridge that is the typical Keys dive bar without the mainland lunatics. Not to mention that the fishing in Marathon is fantastic. Faro Blanco link below.
Have fun and stay out of trouble!
Faro Blanco