I just picked up 20 sheets of
CavityRock from Roxul for 96 bucks (Cdn). It doesn't have the absorbtion numbers in the product liturature because it's not being marketed for that, but it is comparable to their RXL-40 board - same size (2' X 4', 2" thick is the stock item) but just slightly denser at 4.5 lbs/ft³. I haven't put the traps together yet so I don't have any before and after numbers, but I'm expecting slightly deeper bass trapping with this stuff.