I haven't tried the cheaper discs, so can't comment on those. I agree it is a huge price difference.
Given how slow my Sat Nav is I couldn't justify spending the £100+ on a Navtech disc. ('Cheaper to get a better perfoming Tom tom', I argued to myself).
Every now and then a second hand disc will appear on E-bay - I got one for about £40 (from memory), a year or so ago.
If you want a very recent edition, this may entail a long, wait, however.
Good luck,
David