1. It won't have Face ID.Just like the ProMax the iPhonebook will start at 256GB.
Compare
512GB iPhoneBook for $2200
to
512GB iPhone Pro + 512GB cellular iPadMini + 3 years of a data plan for the iPad
It's hard to say which slab phone will be its closest analog price-wise because it will probably have the iPhone Air's cameras, but it will have the Pro Phone RAM and at least the ProMax's battery.
But yeah, if you keep one or both of your device base then the iPhonebook will be more expensive.
2. It won't have multiple cameras.
3. It will have a short and fat aspect ratio.
In no way will it be a Pro iPhone. It does not compare. It will be a worse iPhone than a Pro or Pro Max. You're supposed to be OK with that, for the privilege of unfolding it into the worst iPad.