About 4 years ago I rented out an office that offered free electricity and turned it into a mining operation. I had a total of 9 GPU mining rigs with 4 GPUs each. Used a high end portable AC along with the building's central cooling system to cool the space, also popped out the ceiling panels for more circulation. Unfortunately the operation lasted about 6 months and I was mostly mining LTC, DARK and other alt coins. Sold off most of the rigs except for 3. After 4 years of inactivity in mining I decided to re-enter the game. Upgraded the GPUs on 2 of those rigs from AMD 280x to AMR R9 480x. Pointed them to ethereum classic mining pool around March of this year. It has been mining away earning about 2 - 3 ETCs a day. Well since then, ETC price has taken off like a rocket ship and the decision to start mining again is paying off.
Here's what I have mined thus far, electricity cost is about $70-$80 a month with the 2 rigs running with 3 R9 480x each.