It is if you have to have the latest 'innovations' and kit but otherwise I reckon it's pretty reasonable. Bikes have been decent for a while so you can always pick up stuff that's a couple of years old or second hand for very reasonable prices.
It's really as expensive as you make it.
From my point of view I have two rather expensive XC bikes, carbon frames, XTR, Formula R1 etc etc. But I save them just for racing, the cost of general wear and tear on them is horrendous and that's before I crash and break stuff, I can't afford to use them for training.
My last race will cost me £160 for a new front rim (pulled a couple of spokes through) £40 for a wheel build (the one job I can't do) £80 for fork bushes + £15 p&p to send the fork lowers off, £50 for frame bearings. £275 of stuff wrecked in one race, and just wear and tear items.
However, I also have a rigid SS bought for less than £500 which I've barely had to spend a penny on since, just brake pads and grips really. Even the tyres are worn ones inherited from the posh bikes.
However, my biggest cost for racing bikes is travel. Diesel is the biggest expense, but add in plane tickets, ferry tickets, car hire at the long haul destinations, travel insurance etc etc and then it really starts to add up. Running is no cheaper, yes you can do it in a £40 pair of trainers but if you get good and start competing the travel costs above soon mount up.