This sounds like general builders used to noise transmission between houses, or rooms, not studio construction. The thing that is happening is you are following the builder, not setting the rules - you wanted Roxul, or at least expected it and you got foam?? Insulating for heat is a foam positive, but for studios, then room within the room is the usual thing. The costings seem very, very strange. In the UK, 12.5mm plasterboard )our term for sheetrock) in 8x4ft sheets (yes, we mix metric and Imperial!) is £7. Your 300 sq ft which for the maths I took as 20ft x 15 ft requires less than 18 sheets per layer - so that's £250 for two layers. Add the ceiling and it's way, way below your prices. For a garage sized space (and ironically I start construction of one on Monday) I'm looking at a timber stud work inner shell, clad with two layers of the plasterboard and one layer of MDF, as there is need for the ability to fix heavy things to the wall in the future. This will also be my first install with strip and isolation clips and these cost nowhere near your quote, and the price I paid I considered expensive. I think this will be my 13th studio build and the guy it's being built for wants these strip and mounts because he has read a lot about them, but my experience is that building a recording studio involves an awful lot of different techniques that together provide little bits of extra sound reduction by reducing transmission. These tiny little 'extras' add up. They also cost. The biggest problem is that all these fiddly and expensive techniques and products get wrecked in an instant by leakage. In the case of the garage conversion I'm starting - which is similar to yours, but slightly smaller - around 250sq ft, the door has already been shut off from the space by a stud wall by the builder who was originally going to do the studio build. Before I start on Monday, We did my usual test of the existing structure - I closed the door and turned on a smoke machine and filled it up, then walked around outside looking for smoke. There was plenty. I'd guessed the roof would leak, and the tiles of course were poorly sealed, but as the roof frames will be sealed from the void above, this doesn't worry me, but the stream coming through the poor cement joints where the concrete lintel runs across the door frame were quite interesting. Investigation with a torch reveals air gaps. Another leak was from the door frame to brickwork join. Signs of some caulk, but clearly no real penetration. My feeling, and I don't know for certain, is that the benefit of using strip and the rubber mounts provides a small additional decoupling from the timber studding, but adding ventilation reduces the insulation, an extra layer increases isolation, and the door frame reduces it. It a fully professional expensive studio, all these things are sorted. In a garage conversion, common sense says there is a limit to how effective the total is. My system, developed from a couple of excellent text books - which often conflict in their advice, is now based on cost effectiveness, and as I am NOT a builder in any shape for form, just a keen self-taught DIY person. Hanging heavy sound proof doors, for example. My DIY is not good enough to hang a door to the accuracy required, so I cheat. I hang the door on the hinge side and then build the surround so I can keep the gap to the absolute minimum. It needs to close onto the soft strip to help prevent leakage, so I then cut the closing strips, fit the foam to them, and then with the door properly shut and latched, I push the foam into the door, make sure it is snug, then fix it in place. In my early studios I wasted so much money on an extra layer of plasterboard and then let the sound leak from the doors. I also discovered another common place for leakage is the ceiling joint with the walls. You normally build the walls, the clad them - you then put the cross members up to act as joists from which you hang more sheet material. I like to have sloping walls to prevent parallel surfaces - which works pretty well, but this then means the wall to ceiling joint is a cut edge, not the factory perfect one. Repeating the smoke test on a previous project shows this join as a potential leak. On the next build I've come up with a solution. I'm going to do the walls, and then sit 18mm MDF on top of the wall top plates and sitting on a layer of glue. I'll then add the joists on top. The reverse of how it would normally be done, but it will seal the potential gap. I can then add the layers of plasterboard. I'll then have a perfect ceiling - as good as it can be and then I'll drill a hole for the lighting cables. See the problem? In home type studios - how far do you take insulation increases when you have to mess it up for practicalities. Builders will see the hole and give out a squeeze of caulk. That's a weakness that if your back was turned you would never know about.
I've been shot down for my approach before by people quite rightly quoting technical specs on materials. I discount many of the complaints because the weak areas totally wipe out the expensive improvements. In this build, I'm also going to use my successful wall treatment. Two layers of plasterboard on the stud work side and the inside finish of the room finished with 18mm MDF. It's structural, so you can put some vertical supports up for brackets for synths and keyboards, it is very tough and you can crash things like flight cases into it with no deep gouges, unlike plasterboard. It's very dense and heavy and resists deforming.
On the green glue front, when you tape a sandwich of sheet material that has it, the noise is different. I wonder if it's really the glue, or the air gaps? I do know that multiple layers do give different results. One studio, which had a large floor area so I could lose a bit more on the wall thickness, I included a layer of light weight insulation board. It has a pretty feeble insulation value to sound, but it did seem to allow panel flex to help reduce leakage at the bass end. This studio had a stud work inner, and a studwork outer, and had a squeeze through (just) gap around the outside. This treatment on inner and outer was better than the garage style inner room, and single skin brick on the outside, which is also in line with published material audio specs.
I'm looking forward to seeing the time difference using strip and mounts adds to just fixing to the studs. I'm going with them as I want to scoop up every little extra reduction I can.
The average DIY person is perfectly suited to studio construction because they will take care and time is less of a cost. My experience is general builders hate the enthusiasts who want studios. A good example is where in one build the roof space was formed with premade, off the shelf roof trusses. To maximise available height the spec required the plasterboard needed to seal it to make the airspace in it separate, to prevent leakage was amazingly badly done. Fair enough the cutting around the timber diagonal struts was fiddly, so quite a bit of it seemed to have been done with randomly cut offcuts. As it was finished when I arrived, I couldn't check how much it leaked because of through gaps into the void. It worried me though.
DIY skills are quite basic. If you can hand saw straight, use power tools accurately, and understand the basics of studwork, it's well within the abilities of a gifted amateur. You can also change and amend the plan as you go to improve things. I've also planned this one to have some membrane absorbers - I've found a design from a source I trust for ones that use roofing felt as the membrane, and have a effective range of an octave above and below the frequency they're built to, BUT calculating in advance the exactly frequency where I get the usual bass peak from the room small size and reflections, I'm designing two that will fit into the roof void. The existing roof trusses are 600mm spaced, on centre. So I'm going to leave a gap in the ceiling and then build a bottom open cube, with the absorbing membrane and insulation inside, and then it can slot into the roof opening. I'll seal the opening with a plasterboard or MDF temporary panel, and do the audio tests. This will give me the ability to adjust the internal volume of the cube to be effective at the right frequency. They also could be removed to give access to the roof space if in the future I had any problems - if the owner wanted extra lights or perhaps even proper aircon, we could get access up there.
Your budget figures seem a bit scary. I'm assuming the price incorporates a lot of labour, because the materials are much cheaper.