The higher priced cards are either given better cooling to allow it to clock higher or they are handpicked as those cards that perform better than all the rest. (Or Both)
"Won the silicon lottery"
Having said that if quality is what you are after than I would be more concerned with the warranty than the price of the graphics card
https://linustechtips.com/main/topic/556256-gpu-manufacturer-warranty-comparison/
Most manufacturers come with a 2-3 year warranty with the exception of XFX, EVGA and PNY offering a lifetime warranty.
"Go with XFX, EVGA or PNY if you expect your graphics card to explode after 3.1 years."
If your goal is to get 120 fps in some game and both the overclocked Geforce 1070 and Geforce 1080 provide the needed fps, then it doesn't matter which one you buy from a performance point of view.
Do note that every Geforce 1070 has 1920 cuda cores.
https://www.geforce.com/hardware/desktop-gpus/geforce-gtx-1070/specifications
And every Geforce 1080 has 2560 cuda cores.
https://www.geforce.com/hardware/desktop-gpus/geforce-gtx-1080/specifications
You can't add cuda cores with an overclock no matter how much extra you pay for a Geforce 1070.
The stock Geforce 1080 will have more overclocking headroom than the already overclocked Geforce 1070, allowing for ever higher fps.
A souped up Prius can only go so fast ...