With regards to the comments that MTBing is not expensive, here's the contradictory view.
Even £300 is a lot of money to spend in one go for some people. And especially when it's "Just a pushbike" as seems to be the most often comment.
I think the "working class rule of thumb" shall we call it is if it costs more than £50 it's too expensive, but anything under £50 (including a night on the beers) is an acceptable expense for a hobby. Yup, fags at £5 or more a pack and beer at £2.50 per pint ain't cheap, but for many they're addicted, and they don't see the cost! Also, they justify it by saying that "it's only a fiver or a tenner here and there, you never notice that really do you, but I can't afford £300 in one go on anything!"
I of course agree with most people's views on here, the average person having just given up fags and binge drinking could afford a very nice bike within a fairly short space of time if they redirected their money, and you don't need a nice bike to enjoy the sport. You can have most of the fun on a £300 bike for sure. And you don't need to spend money going places to enjoy it either.
FWIW, my main bike cost me less than £1k. It's worth more cos I was shrewd when it came to buying the bits to make it by sourcing them 2nd hand off ebay or from here, or from Chain Reaction for the new parts. Of people I ride with whenever/wherever, my bike is usually worth significantly less than most people's. Who's having the most fun? Well, that's hard to quantify. I think I'd have less fun if I was worried about the depreciation I was subjecting a brand new £4k bike to though personally if I had paid for it out of my own pocket.