Perhaps the issue isn't so much the price of boxsets (be they tv, movie or music) but more the over-pricing of single feature films. I have paid more for a 2 disc special edition of a film than I have for a 4 disc TV series boxset. VHS tapes cost more to manufacture but cost a lot less to buy than the DVD equivelant (though few VHS are sold now I expect)

I think that £15-20 for a boxset of a TV show is reasonable, but £15-£20 for a single disc film is not. I try to buy mine from the US or Canada when I can because they tend to be a few £ cheaper, but still more than they fairly should be, the same goes for CD's.