Nvidia is pretty consistent in their raising of prices, by about 200 dollars.
the 1080 is a 500$ Card, sold for 700, when compared to the other first gen card on 28nm (GTX 680 IIRC). Titan OG was "a cool grand".
The 670 launched for 400 dollars (1070 is +50 $), the 660 launched for 230 $ (1060 FE is +70 $). Is it inflation, or is it the fact that these cards can clock to 1.5x of the competition, use 1/2 of the energy in idle and have a pretty good driver track record.
These cards launch and prices decline to about half within a generation, basically x amount of time to get your value out of them. We probably won't see a 4-5 year node cycle this time, but we also won't see 16ff gains next time around.
Card usefulness at those prices is tied to resolutions you are running, 4K single monitor is obviously pointless, multi monitor and "multi projection" depend on dev support.