I see some G5 quad cores listed for a few thousand USD on ebay. I wonder if there is a consensus on a market price for a very good example in 2026? Assuming a used but functional and clean example, what would you consider to be a fair price?

Depends on where you are but I wouldn’t consider anything G5 for over 150€.

For the dualcores that make sense with MorphOS 100€ would be max.