How much (on average) would it cost for sound proofing an average size room?

It is really difficult to state an average cost for sound proofing any room. The cost depends on how sound proof you want the room and what materials you choose to use as well as whether this is a do it yourself project or you have it professionally done.

The price of sound proofing can vary greatly depending on the materials used. For a mid level soundproofing job you can expect to pay about $10 per sqare foot.