On the vaguely related subject of "Why are some things so ****ing 
expensive?"

Neat Japanese exercise book* : 9 quid
Accompanying cassettes : 43 quid

... what the heck?  Incidentally those cassettes are required for 
about 1/4 of the 9 quid book's contents.

Now no doubt you could say something about the possible 
greater expense of audio material so here's another statistic.

2004 past JLPT questions level 1 & 2 with two CDs 
included : 14 quid.

Now I'm not going to say "It's my right" or anything but I'd 
be pretty darn tempted by .mp3's of the audio material for
that book.

* 凡人社：１級問題集：日本語能力試験対策