C.Ann
Marathon is approximately 50 miles from Key West. Ft. Lauderdale is approximately 200 miles from Key West.
This has been about 10 years now, but when I researched hotels in Key West I found a Days Inn right at the entrance to Key West that was the cheapest place to be found. If my memory serves, it was your typical Days Inn, nothing fancy but serviceable and clean.
I'm sure there are probably some new hotels by now. We were just there this past July 4th, and I saw that the Days Inn is still there.