A maxed-out MP from Apple rings up at an absurd price tag of over $50,000. If I bought the upgraded parts myself (especially when the prices come down eventually) and hired someone to do the upgrades, there's no way it would cost remotely near $50k. Getting the best bang for the buck is the key here.
Intel® Xeon® W-3275 Processor (38.5M Cache, 2.50 GHz) quick reference guide including specifications, features, pricing, compatibility, design documentation, ordering codes, spec codes and more.
ark.intel.com
The top end CPU for the Mac Pro runs for about 4,500$ USD by itself.
Easily upgrade the memory of your 2019 Apple Mac Pro up to 1.5TB. OWC memory kits are 100% Apple compatible and backed by a lifetime OWC warranty.
eshop.macsales.com
Getting full 128 GB RAM slots for your system's RAM runs for about 12,300$ This is where the majority of your savings comes from in a custom build. Apple supplied RAM is way more expensive, but also is free to replace should it go bad. Apple is not going to replace a third party's faulty RAM. You'll need to weigh the two options to determine if the markup is worth it to you. For me, it was not. I saved a boatload by buying third party RAM.
Upgrade or expand the graphics in your Mac Pro with the Radeon Pro Vega II Duo MPX Module. Featuring two AMD Radeon Pro Vega II graphics processors, each with up to 14.1 teraflops of compute performance, 32GB of memory, and 1TB/s of memory bandwidth, the Radeon Pro Vega II Duo MPX Module is the...
www.apple.com
If you want to run the Vega II DUO (which is really two GPUs on the same MPX module) then those run for 5,600$ ... each. You can get two of these. Or you could alternatively go with whatever top end rendering AMD card Apple supports in the future. Either way, the price will be about the same for the same number of GPUs.
The 8TB SSD Kit for Mac Pro enables you to upgrade the internal SSD storage capacity of your Mac Pro. This kit, containing two 4TB modules, replaces the current SSD module(s) in your system. Installation required.
www.apple.com
Since you are limited to Apple's proprietary SSD kit, you need to upgrade from them. The price on these items will not depreciate much if at all from Apple, based on the price of the 2013 Mac Pro leading up to the 2019 Mac Pro. That kit will run you 2,800$ to upgrade to.
So going from the base model to the mega max upgraded model would cost you about 30,800$ before labor. This is before you consider that you void the warranty on such an expensive machine by replacing the CPU yourself specifically.
Alternatively, you could shell out for the top end CPU in your first build, at the cost of 7,000$ USD (as opposed to the 4,500$ aftermarket part with questionable compatibility). This keeps your warranty intact, and your system still fully upgradable.
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I'm also curious what you're doing that needs massive parallel GPU power, and massive parallel CPU power at the same time, while using 1.5 TB of RAM. Even the most demanding server workloads today don't use all three simultaneously, so anyone who looks at the 50,000$ Mac Pro as an absurdity confuses me since that machine's work case doesn't exist to my knowledge. You generally only need to max out one "area" of your system that is bottlenecking your workflow, as opposed to cranking all of it.