It depends on the manufacturer of the motherboard.
If it's a Chinese remanufactured board that we see Youtubers play around with, stay away. I haven't found a single user experience where the board didn't die within a year.
If it's an enthusiast board, like an Asus or Gigabyte, then about 5k is a good price point considering how old the platform is.
If it's an enterprise motherboard, like Supermicro or ASRock Rack, then you could pay 1k or 2k extra since those are very reliable.
AMD's Ryzen completely obsoleted any Xeon between upto v4 in terms of performance, the only advantage the Xeon has is memory capacity.
You could get a Ryzen 4500 with a A320 and single thread performance would be 30% higher than any Xeon upto v4.