Electricity costs in the UK, average 14.37p/kWh.
If a GPU uses 100W extra, on a 95% efficient PSU, it draws 105W extra from the wall.
If you use it 4 hours a day, 5 days a week, that's 20 hours a week, or 1040 a year. That's 109.2kWh a year, or £15 a year.
If you keep the GPU for 3 years, that's an extra £45.
Given that we all know the price won't hold, it will end up being more than the 1070. Add in the extra electricity cost, and it's quite a bit more, to the point that the "Gsync tax" is smaller than the "AMD tax". You also potentially need a bigger PSU, the thing is over a year late, etc. This one is a bit of a fail from AMD.
If more performance were on offer, maybe, but the vega 64 uses more power than the 1080ti, costs as much as the 1080ti, and performs way worse. It's a pointless product.
Hopefully with Navi AMD will be an option again.