So while I'm currently in holiday in Greece, I still need the forum and some random research to keep me running.
This is why I made this little table.
The order is, Graphicscard, F@H PPD, Power consumption in W, PPD/W, average price on eBay in Germany, PPD/W/Price
GTX1050Ti
180k - 75w - 2,4k/w - 130€ - 54
GTX1060 6G
380k - 135w - 2,8k/w - 200€ - 71
GTX1070
650k - 160w - 4,0k/w - 370€ - 93
GTX1080
750k - 185w - 4,0k/w - 500€ - 125
GTX1080Ti
1300k - 280w - 4,6k/w - 700€ - 152
So while the GTX1080TI is the most expensive, it also performs the best per watt. The performance sweetspot is held by the GTX1070 and the GTX1050TI is the best if you don't want to spend much initially.
I wonder if Nvidia is willing to sponsor me 4x GTX1070 for a private server?