foam sound dampener blocks aren't too bad price-wise either, I hear.
I knew one guy who was describing his sound insulation in an apartment. IIRC, (this was some time ago...) he had plain blue foam insulation against the walls, then some of the dampener blocks, and big curtains / carpets over that. He couldn't crank a Marshall 100 stack, but he could get well above conversation level without bothering the neighbors (he had previously informed them of what he was doing and solicited comments.)