Way overpriced for what you are getting. A new BTO iMac is a lot less. It has an updated display, higher turbo boost and a considerably better GPU.
iMac 2020
- Updated 5K display
- 8 Core i7
- Radeon Pro 5700 XT 16GB
- 72GB RAM (If you upgrade yourself)
- 2TB SSD
- New with 1 year warranty
- 10GB Ethernet (If you need it otherwise save $100)
- $3,710
You'd have budget room, if you want. For the 10-core CPU and Nano Texture Glass or 4TB SSD.
All the iMac Pro really offers is two more Thunderbolt ports. Technically you could upgrade to more RAM and a higher core count CPU some time in the future but the iMac Pro is a sealed unit. The iMac Pro offers better cooling too.
When it comes to electronics. If buying used. I'd never spend more than 70% of the new equivalent. As the iMac I spec'ed runs $3700 that would make the iMac Pro $2,600. Of course plenty of people will spend more. I just don't think it's worth it.
Going by used prices on eBay. That iMac Pro has a used market value of around $3,800. I found two sold listings for $3,400 with 8 core CPU, 64GB RAM and 1TB SSD. So, I figure an extra $400 for GPU and SSD upgrades. Still the new 2020 iMac is a better deal in my opinion.