I have used a relatively cheap material for my HT room. That material is the Superlon's sheet (the same material that they used for refrigeration or air cond ... those black spongy thing)

I am quite happy with the result for my HT room... in term of sound and also asthetics.

I bought them from an air-cond supply shop in Klang. Superlon sheets come in many thickness. I bought the 13 mm thick, 3' x 4' size. Cost approx RM15 per sheet.