It depends on how handy you are - or you can get somebody in to do it - but I found that fixing one of the gyproc thermaline products to the wall (they stick on with plaster-like adhesive) and plastering / decorating over worked very well. I filled the bedroom alcoves either side of chimney breast in this way.
http://www.british-gy..._thermaline_plus.aspx
Try one layer - if you wanted to be extra sure you could then fix an off-set layer of ordinary fire resistant plasterboard on the top of that, and that should baffle the noise that isn't transferred through floor joists.
I agree that the products marketed for domestic soundproofing are very expensive, and probably overkill for the solution you want.