You are missing so much.   You need to know your electricity cost, your equipment (cooling, psu's, etc).   Each can very depending on location and that brings up cost of location.  But unless you are hosting somewhere you have a lot to figure out.  And giving you ROI months out.... no one can tell you accurately especially missing so much info.

It's going to be highly tied to electricity price.  But most will be looking at S9 that are buying new gear due to specs.