Best Cheap acoustic wall treatments?

Yes they can be made very cheaply. All you need is some wood for frames for the 2 feet by 4 feet Owens-Corning 703. Burlap is a cheap fabric that can be purchased in a few colors and is acoustically transparent. The best looking fabrics as well as performing are the ones from Guildford of Maine. I used GOM 2100 series in black for my panels.Try www.fabricmate.com for great prices on GOM. My wife complained about me making stuff until the end result. If you take you take and make sure the frames are straight, when you cover them with fabric, they look as nice as the commerically made ones. My wife likes them and says that they look good. I used Z-clips that you can get from www. rockler.com for about 12.95 to attach them to the wall. Unlike many tweaks, this one will be very easy to hear.

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.