Mac Pro and Mac Studio, spec'd to the same maximum possible Ultra cpu, 192GB ram, 1tb ssd, Mac Pro is $9600, while the Mac Studio is $6600. How many people really need the Mac Pro's PCI-E expandability (which probably no third party GPU's can use) to justify the $3000 premium, in an arguably worse form factor?
The amount of customers who really need PCIe slots are so low that the number of sales to amortize the R&D on is way lower, hence the premium.
The people buying the Mac Pro are probably all going to be high-end video and audio professionals for whom the price difference isn't as noticable
edit: one of the PCIe cards they showed was the Avid Pro Tools HDX card which costs $5000 alone. People who need that card are the target market for a $3000 PCIe chassis.
