PLENTY of those still available 2nd hand in rather good condition. As well as Transalps and older Teneres. DR Bigs becoming more difficult to find in good state, but still available. Old Superteneres same.
KLE500s chip as dirt here. I personally do not like that bike - in my opinion in stock state it kinda...not nice. But it is easy to modify.
As for Africa Twins - have not really noticed shortage of those, but for good state you will pay. Still no more than half of new CB500.
Hell, even more rare things available. Last year my friend picked MINT condition Aprilia Tuareg Wind 600 in Germany for about 1200 Euro.
Take a peek at today's list of offers for KLE in Germany