1x1080ti 2018 = 5x750ti 2014.
Cost to build a "rig", earnings, power consumption, everything. Well except those free pci-e slots you theoretically have today.
That might be true for some algo (some memory intensive ones), but it certainly isn't true for those that make little use of ram. Pascal is 16 nm and Maxwell is 28, that means more transistors and less power.