Hmm - I'm having some problems finding anything under $400 also. Your travel dates are easy for me to remember because we are going Oct. 16-26! The reason your rates might be so high is because you do hit Thanksgiving during your trip and I doubt there are going to be many discount codes during that time since it is a big $$$ time...I could be wrong though.
Have you checked Thrifty? They are not onsite at the airport, but you could send the driver to pickup the car while the rest of your party collects luggage and then the person picking up the car can just pick you all up at the airport. Plus their rates are the lowest I saw.
Try using corporate code 004M001010 from the Orlando Magicard. It gave me a total of $401.31 for your rental. That is the best I saw.
I do agree with the pp to be patient and check rates often...you may see a drop. Book now in case they go up though...it's such a game.
Good luck!