Are you trying to ask how an external graphics card would compare to the Radeon Pro Vega graphics chip in the iMac Pro?
The answer to that really depends on how locked-down Apple makes it external graphics card solutions. Right now, based on what we know of the developer kits, Apple is using a Sonnet external enclosure, which should be able to take any type of card that can be powered by a 350W power supply. If the sky is the limit in terms of your graphics cards, then it's very likely that you'll be able to buy something more powerful than the Radeon Pro Vega, which is considered a high-end card (but the exact card type used in the iMac Pro isn't shipping yet, so direct benchmark comparisons will likely need to wait until December).
Trying to find comparisons, some have suggested this card will challenge Nvidia's GTX 1080i (~$600-700), while others have said it is meant to be at the level of the nVidia Titan series of cards (Titan XP has a MSRP of $1,200).
And of course, graphics card development is pretty fast... so the prices above only really apply if you'll be trying to match the iMac Pro's graphics chip immediately, and assuming all cards will be compatible with the external graphics solution. With time, faster cards will come out, and today's fastest cards will drop in price. The iMac Pro will be capable of using external graphics, too, but the Radeon Vega Pro will eventually seem outdated compared to what can be run in the external solutions.
This all assumes, again, that Apple doesn't cripple the external graphics solutions, either by locking it down tightly in terms of card selection or through some other less than optimal performance hampering.
