stay away from the foam crap, it does not perform as well as rigid fiberglass or rockwool, which is what the main companies use in their products.
If your even a little bit handy, you can make your own panels a heck of a lot cheaper than buying them. If your not a little bit handy, they GIK and many others are fairly affordable.
On my website I have pictures and articles on this subject. I just called a local insulation distibutor that sells to contractors for heating/cooling, etc. They sell Owens Corning 703 rigid fiber glass panels in 2ft x 4ft x 2" or 4" thick material. It is very easy to work with and cut if needed.
You can stradle the corners for bass traps (low freq's) with at least 4" thick material, and use 2" for the side wall/ceiling reflection points (higher freq's). Of course you will have to build a frame out of wood and cover with some material to make it look nice.
