Don't hold your breath, it will be 600-700 Euro in Europe, at least in the beginning.
No, the definitions won't change. Mainstream is what the vast majority has. It will either be the 1070 or an AMD card, considering their plan to go strong into mainstream VR. I see no reason why people with 1070 would think their cards are too weak to try VR.
Hey, whenever you can, send me the model of the crystal ball you own so I can look it up on ebay.