In Ontario (where I live) the prices were generally quite a bit higher for used ones than what you're quoting...
$17,000 Jetta GLS for a '98 with 106,000kms
$20,800 Jetta GL for a '00 with 90,000kms
Those are asking prices, There was no way I could get the '00 below about $19,000, and I pushed HARD.
I ended up buying a 2002 Golf GLS with cold weather and CD player for $21,500. It had 3,600 kms, (basically full warantee.)
So I'd say the prices you mention are Ok, but not great. IMO used TDI's are overpriced, I think a car coming in off lease should be no more than $500-1000 over the residual value. In other words a '99 with 80-100,000 kms should be about $13,000. Not $20,000