UK pricing is £1099:
http://www.geforce.co.uk/hardware/10series/titan-x/
It's hard to find a GTX 1080 under £600, but relatively easy to find a custom factory OC one for < £650. That'd make the pricing mark up around 70% for a 25% increase in performance. Or to put it another way, you're paying up to around £300 just for the privilege of saying that your card is the fastest single GPU gaming card on the planet.
That said, the 1080 performance @ 4k is exactly in the range where a 20% improvement is very noticable - it's not like it's giving you 125fps instead of 100 - it's giving you 60fps instead of 45 - 50. That might be enough to persuade some people with deep pockets. After all, it's cheaper than SLI 1080s, and I've seen those kinds of builds doing the rounds...