If the next gen Nvidia GPU's are based on the Volta architecture, I would expect higher prices due to various factors such as supply logistics and increased manufacturing costs from moving to a smaller process architecture. If that's the case the new cards will be more efficient for mining, but shouldn't make the current Polaris lineup obsolete in terms performance/hash and ROI timeframe.
Maybe they will not make the Polaris line of RX series obsolete but they surely will be the best choice for mining as they will be with GDDR6 memory which is supposed to be twice the performance of GDDR5.
The price will be really high in the beginning as they will try to market the GDDR6 as the first manufacturers of a graphic card with such memory. If you can find the 1080 ti even used below 600 dollars price you should buy them.
I thought it was memory bandwidth, not speed, that matters most for e.g. eth mining.
