put all costs on the paper first.
1. power price kWh ( let says 0.13 usd kWh)
2. gpu costs... let says nvidia gtx1070 350eur each
3. CPU + mobo + hdd + ram + PSU
put info in mining calculator like whattomine.com... 6 gtx1070 can get around 180mh/s so with this prices its $6.5 per days, of $4.5 with power cost because it will draw around 700W.
so rough around 3000USD you need to build 6x gtx1070 + mobo etc.
in best case scenatio you will earn $1500 per year so you need like 2 years to pay out this equpement. and this prices sis for used items, new are more more expensive.
The real deal is difficulty, ASICs for Ethash are coming so watch out
what you see in online calculators today might about to change tonight, tomorrow or in the coming days or months
Profitability is not constant in mining
of course, just saying, as for now, so best case scenario will be 2 years for return... and in real life, comfort ETA is 3 years....
is it worth, you need to see for yourself.. just to say this... i went for 2 weeks on vacation.. putting my rig off, due to unable to control it efficiently in any case of trouble,... and when i got back,, lot of things has changed... price dropped significantly... and what was profitable 2 weeks ago (lets says Equihash for nvidia, today not so much..things can change every day..), is no more profitable... so estimations are very risky.