Thing is, although they are cheap to buy now, they still cost the same to run (in fact, slightly more) as they did a few years ago.
Mate of mine has a 2001 230K Avantgarde. He bought it about 3-4 years back with 60k on the clock for £4k, so I don't think that they've dropped massively since then, just another couple of thousand which is roughly what you'd expect?
Still, it's had it's fair share of issues - engine wiring loom full of oil, lambda sensor, sensor for the ESP system, gear selector module etc.
New keys are £200+ as Howard says, tyres are still £300-500+ a set, services cost the same, etc.
Not 'cheap' to run if you are unlucky, but at least they are cheap to buy and won't really depreciate significantly any more. The trick is to buy a good one now, run it for very little and sell after a couple of years - and be lucky
'Cheap' cars don't always work out to be the cheapest in the long run, sometimes worth paying a bit more for one with little/no rust, no faults, two keys, good tyres etc.