The iMac Pro is scheduled to be available in December. While the base model has been announced at $4,999, there have been no announcements about the pricing of customisation options, although some have estimated that a maxed-out iMac Pro could cost over a whopping $17K! Upgrade options are likely to include an 18-core processor, 128GB ECC memory and 4TB SSD drives.
As you point out though, once you start speccing up a high-end iMac, you do get very close to £5,000, so you’re right that there will be a dilemma there for people that can afford a Mac of that calibre.
High-end iMac or base model iMac Pro? If you can splash the cash, it’s a tricky choice.