Yeah but no one will bother with 4$ per Day, considering u can't use ur PC anymore.
Having separate rig maybe is worth to mine then. But for example now 980Ti is around 400Eur, so 82.91Eur per month is not really worth it, that's almost 5 months to get back the $ from the investment for GPU's and also when buying used GPU you never know if the GPU was taken out of the oven ( a trick to revive dead gpu for short period of time ), then the GPU could die and u end up in a huge loss.
1080Ti prices went up like up to 1800EUR ( 1000e for used ) and it's around 120eur per month ( almost a year to get back the investment and again the question if its gonna last that year )..
I think instead of investing for mining hw better idea would be invest into crypto trading, faster and risk is nearly the same.
Because current GPU mining profits is very low, except for the millionaires who has like hundreds or thousands of 1080Ti's and other similar cards ( like these rich chinese guys ).