Add $100-150 to the 1080 and cut the performance on the 1070 by 10-15% and that would make sense. Plus you know they will do something to gimp the 1070. No idea about OC headroom.
That would make Geforce GTX 970's successor only 25-30% faster. Unless they run into a severe BW bottleneck I'm expecting a bit more.
You may be right about the cost, people will pay Geforce GTX 980 Ti prices for better performance than GM200 and the newest architecture @ 16nm FinFET, even if it's not the big Pascal most enthusiasts want just yet.