When List of the Dead was placed on pre-order it was originally priced at £3.99, but not long after it was actually released the price dropped to £2.99 (and the equivalent prices in other regions). This was in line with the long term plan of having the initial volume in each story priced low, and a uniform price for all subsequent volumes.
With hindsight, that was not really very fair to the die-hard fans who pre-ordered LOTD.
So for The Ravening Deep the exact opposite is going to happen, which will compensate those die-hard fans.
TRD will be on pre-order for just £1.99, and available at that price for a month after release. Then the price will rise to £2.99 to match that of LOTD.