That's not surprising to me at all. It does have two of the main rules and Warhammer Quest is one of the two big boxed games that GW doesn't have available electronically.
I've seen the main game itself go for $150 to $200, and even higher for an unopened box. So low balling it (based on going prices I regularly saw a few years back) at $300 for the the two boxed sets, about $50 for Catacombs of Terror, about $30 each for the character packs, and about $50 total for the three Treasure packas, puts it at $550. That comes out to about £340, so right now I think it's actually kinda low. If it had some of the rarer more sought after character packs, I'd expect this to be much higher. The Bretonnian Knight, the Chaos Warrior, and the Elf Ranger go for much higher than any of the other character packs. I think most people don't pay high prices for these so much to get the figures as they do to get the rules as well.