770 released at the same price as the 670($400) - just $100 cheaper than the 680.
I think the thing to think about is just "How aggressively priced is a certain card?" In other words, does it have a lot of room for a price drop with what is expected over the next year or so? The 970 was a great deal from Day 1 and held its value basically its entire lifecycle as a result. There just wasn't room for a price drop and there was nothing for Nvidia to replace it with til this year.
From what I'm seeing, neither the 1070 nor 1080 look to be 'aggressively priced'. The 1070 is still cheaper than the 670 or 770 was, but not by much. And both those cards could be found much cheaper a year later.
If you really want the best value, I think it'll be worth waiting this time.
It's also possible AMD bring out a smaller Vega card next year that competes with the GP104, motivating a price cut even more.