You can stop some sound "leakage" with cartons and carpets and anything else but you're not going to stop a lot and you're really not going to stop any of the low end freqs that rumble the neighbors. To stop those you need a lot of limp mass and tuned traps in combination with a floating floor (if you're on a slab).

Just keep in mind that for the money you guys are looking at there is no such thing as "sound proof"

Honestly, I wouldn't even bother worrying over it or spending any money unless you have specific issues with the sound on the inside and you could benefit from difussion or isolating a precise set of freqs that need to be tamed -- but even here you're going to need some analysis tools that you probably don't have.

I researched studio design for two years and read book after book, article after article on all this...unless you have thousands of dollars to spend on serious redesign of the rehearsal space then you're better off not bothering because you're just throwing money away on nothing if you're trying to get sound isolation. Nothing you're going to do will get an air-tight room and a dead-air shell around that room and as long as there's even a small "hole" in an otherwise air-tight room then all the effort is for nothing.

And a $10,000 budget to create a "sound proof" studiorehearsal space is about what you'd need to spend (variable depending on how much of the work you do yourself) and still you wouldn't have a "sound proof" room.