Well, it's a lot if you're not gaming. I think it's more the fact it's the jump from £10 to £25 for a so called "gaming mouse", when those unfamiliar with the role of a gaming mouse can't really see the difference, except perhaps a few higher numbers.
I couldn't justify much more than £30 to be honest, while i can see that the G9's, Mamba's and X8's of this world are very nice mice, given money is very much an object I'd much sooner put the money into hardware, however I would say that the jump from an intellimouse to a 518/Abyssus etc. imo is much more worthwile and much more significant than the 518 to Mamba.
Although, that said, I'm not a particularly serious gamer, nor do I have the skill to really be able to make the most of those mice.