Note that it should be possible to add RAM to the iMac Pro though it is not easy:
How to instructions and video to replace or upgrade the RAM in the iMac Pro models (A1862). Actual max iMac Pro RAM, too.
everymac.com
Both the iMac 2020 and the iMac Pro 2017 have a lot going for them. The iMac Pro 2017 higher end for it's time so includes a lot of nice "pro" features like ECC RAM. The iMac 2020 3 years newer so its CPU are of a newer architeture and slightly faster even though relatively lower end.
It will be interesting to see if macOS keeps support for one over the other in coming updates. From an engineering perspective there doesn't appear to be any benefit to dropping support for the 2017 iMac Pro before the 2020 iMac. In the meantime, I suspect the iMac 2020 is commanding a premium over the iMac 2017 Pro due to the vintage and uncertainty around support even though the latter should perform +/- similarly.
Back to the original question, if I was a seller I would probably start at the $1500 that Mr. Fox threw out and see where it goes from there. Two limitations of checking ebay for market prices -- by default it shows the prices of things that haven't sold (whereas e.g. Bringatrailer has a historical price page) so it is biased towards higher prices at which the items may never clear (as opposed to lower prices that came and cleared). Also as sellers we tend to look for the highest prices we might get for our favorite things. As buyers we're more likely to sort prices low-to-high and perhaps even comprimise on some of the specs looking for that deal. Alternatively buyer might pick a few models that seem closest to our item from an
eBay buyer's perspective and follow those specific sales through to the end to see at which price they clear and how long it takes.
P.S.Established sellers with a reputation also command a premium for the typical Ebay buyer so would also consider that as well as inclusion of original packaging and other non-functional specs.