Best Cheap acoustic wall treatments?


I am converting a 12x20 space into a home theater and want to treat the room. The walls and ceiling are drywalled, with thin carpeting on the floor. Are there any inexpensive wall panels or the like that are available? Should I consider draping the entire room in fabric? Thanks for your thoughts.
jmslaw
Does anyone have a source for the Owens-Corning 703 material? I haven't seen this at Home Depot. Is it a styrofoam-type sheet, or compressed fiberglass?
It is a compressed yellow fiberglass.Do a web search at Google in your area.The lumber yards usually won't carry it.Try "insulation" in your yellow pages.I think I made (16) for about 250.00.Fabric incuded.I also user a thin plywood as a backing to staple the fabric to.The plywood also makes it easier to hang on the wall.
You are right, Home Depot does not carry it. If there is a Lowes in your area, they carry it. Otherwise go to the Owens-Corning website and do a search for dealers in your area. It is similar but, not the same as the rigid insulation that Home Depot sells.