Is this a good price for Mineral Wool?

I am in the research/planning stages of building a 4' W x 5' L x 7' H (interior dim.) iso booth that will be used mostly to practice and record ac & el guitar, percussion (congas, bongos, cowbell, etc), and vocals/voice-overs. I'm hoping to avoid bothering the neighbors (and my wife!) and to be able to get dry, relatively noise-free recordings. I'll be putting the booth in my apartment for now, but my plan is to build it in such a way that I can disassemble it and take it with me when I move into a bigger place. The apartment is on the 2nd floor of a 3 story building with neighbors above, below, and on 3 sides. My budget is around $500. I have no construction experience but will get some help from people who do. Anyway, I really wasn't planning on posting here until I put my thoughts together more but I found a place in my area that's selling Roxul RHT 80 Mineral Wool Insulation for $6 per 2" x 4' x 6' sheet and I wonder if anyone can tell me if this is a good price. I know I could/should do the research myself and I'm in the process of doing so but if it is a good deal I need to buy it before it's gone. Apparently this place has "a few pallets of it" and I don't know fast they will move it. I'm planning on using an inside out wall design and I'd use this stuff between the studs. QUESTIONS 1) Is Roxul RHT 80 ok to use in the cavity between the studs in an inside out wall design? 2) Is $6 per 2" x 4' x 6' sheet a good price for Roxul RHT 80 Mineral Wool Insulation? 3) Would I be better off using some other type of insulation? Cost is definitely a big factor here. I'm trying to save wherever I can. P.S. I just found this forum last week and my head hasn't stopped spinning! Thanks to the administrators for providing such a great resource and to everyone for sharing so much knowledge and experience.

This is what I got Roxul AFB for here in MA: 3x15x64.95 - $25.60 /64.95sqr ft = .39 psf 3x24x64 - $21.76 /64sqr ft = .34 psf each bail had like 8, 4' batts

Thanks ltata. So maybe .25 cents per square foot is pretty good. Of course, we're not comparing exactly the same product. I don't know what the differences between Roxul AFB and Roxul RHT 80 are.

Maybe this will help - http://www.roxul.com/sw34127.asp

Well, I'm not very familiar with how to look at these specs but it seems to me that Roxul AFB and Roxul RHT 80 are very similar in acoustic performance. I need to learn more about interpreting this stuff though. I'll also need to compare specs and cost of mineral wool vs. the pink fluffy stuff. I seem to recall reading that cheap, regular insulation (pink fluffy stuff) was also fine for this application.