The couple places I found some it was about $570-$860 per 64Gb memory module, so ~$2280 to ~$3450. NewEgg is having a sale, looks like real street is about $20

The couple places I found some it was about $570-$860 per 64Gb memory module, so ~$2280 to ~$3450. NewEgg is having a sale, looks like real street is about $2000 if the modules they carry are compatible. But officially, for RAM upgrade, the iMac Pro is not user-serviceable, you have to take it to a service center to get the memory upgraded -- and if you go to Apple, they will probably not install 3rd party RAM.