mining zec
1080 = +- 500usd = `500sol
1070 = +-400usd maybe more= `400-440sol

buy 7970 aka R9 280x =150usd maybe 130usd = 330sol
500usd 3x card `1Ksol =profit, volts tuning 1.081 =180-190Watts
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And show me a game that would not draw the old graphics card 7970(280x)
GTX1080 gives about 570-600 sol/s at 170-180W power (OC, DV)
GTX1070 gives about 450-480 sol/s at 140-150W power (OC, DV)
And buing new GPU you get a 3 year warranity, but buing used 7970 or 280x it can be broken in few days and you will get nothing
my R9 280 working at 2013? and dont broken, If buy today needs looking the state of the video card, so that there is no smell of textolite, then it is understood that it did not bask more than 85?and coolers state. Profit is achieved faster than expensive cards.But this is a personal opinion