DIY vs BUY - Soundproofing
Portable Floating Floor
I hear so many people stressed about floating a floor in a non-permanent studio. In most cases this is not an audio necessity! You only technically need to float what is being used. Assuming this is a project studio and you aren't trying to impress everyone, why hassle with a massive and expensive project like that?
I feel pretty lame even posting this, but here it comes... The one good thing that I have to say about Auralex. They make a "Great Gramma Floor Isolation Platform". I am not personally saying that it is great, that is just part of the name. It is meant as a riser to keep low frequencies from transferring through your floor from speakers or amplifiers. I bought a bunch used off ebay who since floated their studio floor and didn't need them anymore. I have found that they also work well for keeping ground vibrations away from microphones, keeping a singer from creating audible noise while shifting during a vocal take (old wood floors) and keeping sound transfer through the floor to a minimum when I am playing percussion or Midi drum triggers. My studio is above other apartments and I have had no noise complaints since I switched to them.
Floor Floats 
I also used the last of my 2" acoustic foamboard behind my Rivera open cabinet amp to help absorb sound reflection off the back wall and hopefully drop the through wall noise as well. I doubt it does a ton, but it probably doesn't hurt things either.