I will tell u my story...  

Just made a ring with 4x 1070 ti of Zotac, the whole system did cost 2.485 euros.
Its card is giving average net profit of 3 euros per day.
The Index of return on equity is at 136%, that mean that for 1 euro u are getting back 1,36.

What you want to be sure is that nothing will go wild down in the next 4 months, (you can sell the hardware and reduce the losses).

For me it worth, but do it slowly.

1st invest 10.000
2nd invest 20.000
3rd invest 20.000

and go for GPU's because of the resell safety.