About $50 per adult.
We had a group of 12 last time. We reserved eight months in advance and I paid right when I reserved. We were seated about 1/3 of the way back from the stage on the right hand side. It was perfect.
We always reserve far in advance, Usually more than eight months. I was worried last time that because I only reserved eight months in advance, I may not get floor seats. Luckily, we were staying AKL concierge for that trip. The concierge CM was able to call over to the HDD place and tell me exactly where we would be sitting. She did this about six months in advance.
I had never heard that anything was dependent on when your tickets were paid for. I try to pay for anything I reserve right when I reserve it. I just find it easier.
Somewhere in the front of this site they have a good review and description of HDD. Also at Deb's site, wdwig.com, also at mouseplanet, searches on google will actually come up with individual threads on this site and others that talk about HDD.