Apple added some new configuration options to iMac Pro today. This includes 256GB of RAM. The $ 5,200 you can see on the 9to5Mac can be added to the computer at a reasonable low price (19459003).
You can buy a second iMac Pro for a single RAM upgrade. iMac Pro offers 32GB of RAM by default when compared to Apple's other RAM upgrades. Upgrading to 64 GB adds $ 400, and upgrading to 128 GB adds $ 2,000. These two prices are really small compared to the new 256GB options.
What's new in iMac Pro is the upgraded Radeon Pro Vega 64X GPU with 16GB memory for a relatively inexpensive $ 700 upgrade. (For comparison, iMac Pro comes with Vega 56 GPU built-in, and Apple previously provided Vega 64 GPU options at $ 550 more.)
All iMac Pro-equipped iMac Pro with 2.3GHz 18-core Intel Xeon W processor, 256GB RAM, 4TB SSD storage, and Vega 64X GPUs for a total of $ 15,699.