Would I pay more for a 6D II with a 5D IV sensor? Yes - absolutely. How much more? No idea - I've gone a different direction and moved away from considering the 6D II.
I had an original 6D which I used and loved. It was a great camera and took great photos for my needs. I take photos of almost exclusively landscapes and nearly always on a tripod. My biggest desires for improvement on the 6D were increased base ISO dynamic range, improved weather sealing, increased resolution, and maybe further improvements in low light capabilities (in that order). I certainly understand that these are not what everyone else needs, and I concede that the discussions on DR are splitting hairs for the most part. However, I did find myself lifting shadows on maybe 50% of the photos I took, and I almost always used ND grads or bracketing to overcome the challenge.
When the 6D II was announced I was thinking I would see maybe 3 of my 4 desired improvements over the 6D, and while this seemed to be true, it was not the 3 I expected! In the end I weighed my options and felt the 5D IV was a better fit, so I picked one up last week.
Since I sold my 6D to pick up the 5D IV I couldn't do direct comparisons, but I will say that my initial reactions for the 5D IV have been extremely positive! I have found the files produced suit my need very well and I have no complaints. Anecdotally, I have found that the 5D IV handled scenes where I had expected dynamic range challenges much better than I expected, so I'm happy.
If the 6D II had the 5D IV sensor I probably would have picked one up instead and saved some money. In the end, however, the 5D IV is a more robust camera which satisfies all my needs and gives me loads of room to grow as a photographer - although there can be no doubt that my skill needs more improvement than my camera does

. Regardless, I'm a happy camper!