Cost always limits you to less than you wanted, that's pretty much a fact of life for all of us. Cost keeps you from using 12-foot thick solid concrete walls, room-in-room, million-dollar consoles, etc - Heating, Ventilation, etc ( HVAC) will be a problem whether you do floated room or not, so still has to be considered. About the only things that work for soundproofing are mass and decoupling. The more of each, the more likely you'll succeed.
One way to come as close as possible while using single wall construction would be to utilize staggered stud walls. This might consist of 2x6 or 2x8 (better) plates and caps, with 2x4 studs on 16" centers outside, offset by 8" and repeated on the inside. 8" plates will allow more fiberglas woven between studs giving slightly better STC rating. An underlayer of waferwood on the outside would increase mass - then, a couple layers of mass (particleboard) on the inside, followed by resilient channel with a couple layers of sheet rock - This combination should get you as good a soundproofing as a DIY single wall can, provided you buy lots of Butyl caulking and caulk EVERYTHING. Remember, for an audio room the goal is an initially hermetically sealed room, then you only break the seal where you have to and only with multiple baffles to absorb impinging sound. A good free reference for most of the info you need is SAE -
http://www.saecollege.de/reference_material/index.html
Click on the STC chart for examples of different wall construction. More is better, I would stay with wood framing for outside walls.
A good carrry-around book is Everest's Master Handbook of Acoustics, available at Amazon.com if you can't find it locally.
Also, the degree of soundproofing necessary depends on exactly what you'll be doing in the room - if you're recording mostly thru electronics, and using electronic drums, the requirements are much easier to meet than if you're recording Nuclear Explosions with live drums. On the other side of the coin, the outside noise levels can be all over the map. Do you live in the country, under a major flight path, near a railroad switching yard, next to a shooting range, under a mountain? Neighbors have barking dogs, like to throw massive parties? Big differences, it all enters into the equation. Pro studios do month-long analysis of location noise, noting time of day, day of week, etc - little guys live with what's there, and try to time sensitive acoustic recordings between the outside chaos levels.
Hope this helped, check out the SAE site and buy Everest's book. Both will help you decide what/how/if... Steve