I wouldn't say you're getting ripped off.. that's top dollar for a 2nd story Cabana which can be expected for opening week AND a holiday week. It is strange how one "magically" appeared though.
I believe that's the standard holiday pricing. I was quoted the same price for July 4th weekend. First floor is $100 less.
I'd make nothing of the "magically" appearing. Who knows how well the system is set up right now. They may have misread the system at first, or maybe they were able to release an additional cabana, or maybe there had been a cancellation.