I really think one of the keys is building the home first. If this is a basement setup, then you have many options available to you. But, the key is not getting the walls done I would say.
The key is having an outline of your room, and making sure that the foundation gets built correctly to accomodate for the need of the higher ceiling (potentially) in this location. I don't think a 12 foot ceiling is really unreasonable.
Depending on your 'cool' factor, you could decide the size of the room and then have a sunken foundation at that point. Basically, a step down of several feet to the bottom of the theater. But, then enter at the back of the room at normal floor level, and walk down. Similar to enterring a movie theater from the back of the theater.
Floating rooms I don't think are terribly expensive. It is expensive if you higher a pro company to do all the work, but if you have good labor or DIY then it is just lumber and the rubber feet that the room floats on.
Pricey? How about 10 to 12 theater chairs at $1K a pop. That's the price of the equipment for the theater right there.
I also see the need for a screen in the 120" wide range which leans me to recommending a brighter projector like the InFocus 7205 which is about half the budget for the room's equipment.
Another $1K on the screen, and you are left with about $5K or so for speakers, wiring, and your receiver. It may be worth it to break some stuff into phases a bit, or start looking know for some eBay/Audiogon deals.
ie: You can find a Velodyne HGS-18 subwoofer which will blow you away for under $1,500 sometimes. It is probably a better way to go for that size room, and a few years old means you don't have to break it in yourself.
I remember when a $12,000 budget meant you could afford half of a decent CRT front projection setup and DLP or LCD were words that quickly got you kicked to the curb.
