Slightly less: the iMac Pro uses the ~$100 AQC107 whereas the X99 motherboard chosen uses the more expensive X550AT2 because Aquantia only launched their chips

Slightly less: the iMac Pro uses the ~$100 AQC107 whereas the X99 motherboard chosen uses the more expensive X550AT2 because Aquantia only launched their chips this year. But yeah, the Xeon they chose is quite a bit slower than the 8-core Xeon-W; the Broadwell equivalent was really the $1100 E5-1660 v4. So -$200 for motherboard, +$700 for CPU, and -$100 for PSU if you're feeling generous. That's still $5k.