We rented a 4 bedroom townhome at Regal Palms resort last year for $600 for the week including taxes. That was President's Week. That particular owner has increased her price since then, but I think if you spend some time researching you will find something. I checked out hotelmartusa.com and found that you can get Fiesta Keys resort through them for $79.85/night including taxes for a 3 bedroom and $85.55/night including taxes for a 4 bedroom. I have not stayed at this resort, but I have used hotelmartusa.com 3 times and have had good luck each time.
Good luck.