Yes, the price doesn't go up in a linear fashion (100 page ones don't sell for 1.50, not for a long time), and not even directly connected with other inflation indices. Or much of anything else that I can see, I've had some at around 200 pages for 7.99 and some at 5.99 in the last year (the US/UK market conversions really skew the results; it's always been 'interesting' how a book published in the UK and sold in the US, incurring shipping costs and at least sales tax (possibly import duty as well) still sells there for less (taking into account the conversion rate) than in the UK, while US books have often been re-labelled at the $=pound rate; the latter no longer happens as much, making US imports often cheaper now than the UK versions).