It's more the excessive costs, carbon fibre frames, space age suspension that absorbs the bumps better, etc. They're made to a much, much higher quality, but i'd never pay more that £500 for any bike, personally, unless it was electric, which defeats the purpose for fitness and I see more as assisting those with injured or weak knees/legs.
Standard mountain bikes range from about £120 to £250 ($156-650). There are high end mountain bikes, ranging about £5,000, of course, but I find them outrageously priced (a bit like those who buy a £25,000 watch over one worth £250). Commuter bikes are roughly between £200 to £500 ($260-650). Mine cost £300 ($390) and have had it for 8 years. Practically saved my life when I was hit by a car and the frame absorbed the impact. Use it every week.
Road bikes are where it gets just as ridiculous. £600, £1200, £4400 ($5700) £12,000!!