Given that Harper Collins is advertising the digital edition as having an RRP of £8.21 on
its own website (though this looks like a reference to the iPhone version), this is indeed a ridiculous state of affairs.
This sort of sloppiness is a result of pricing structures chained to SKUs and it needs to go. No doubt there are 'problems' all the way down the chain with the idea of an SKU whose price will vary, but those are just a symptom of laziness.