Roughly £200 or so, give or take £50. For a lot of bikes.
Don't get me wrong, I can see how rare things or exotica can be several hundreds, or thousands.
For myself, a couple of years back, I bought a solid, mid range bike, from here (my 91 Diamond Back Apex) for (from memory) around £110, plus very reasonable costs for postage (£12-15 I think). It was in very good condition, no rust, the odd stone chip, or slight bit of chain suck (just down to primer, and very slight, really). Wheels, gears, brakes, shifters and levers, were all in exceptional and pristine condition (I mean how you'd expect them to look after you'd spent quite a while cleaning and restoring them). And that was all DX throughout (as stock). In fact the bike was in a lot better condition than I thought it'd be.
Now I know there's some variance, and bikes at the higher end of mass-produced ranges can maybe command a little more - and of course there's supply and demand with some models that appear to be in great demand.
So I can understand a very sought after, albeit mass-produced bike, dripping in XTR and other boutique bits, in very good condition, maybe able to command several hundred, but outside of that, I'm really quite puzzled with some expectations.
Normally that's just a sign of me getting (gotten) old and a bit Rowley Birkin, but I can't help but think, in a Clarkson-esque manner, I'm not wrong?