If I can make $1 per day off of each GPU card (profit after electricity and other expenses), then 90 cards make me 90 a day which is 2700 a month which is pretty close to my goal. 90 cards = 15, 6 card rigs probably running in the range of 1200 - 1400W per mo each.
Not really because there are extra costs as you increase. Cooling, space, internet requirements and I'm not even talking about the legal stuff like licensing,taxes, insurance. The cheap mining model works at home on free internet and up to a few GPU's. You've obviously never mined if you don't know this. After one rig you have to start cooling. After two rigs you have to move out of the room. At 90 GPU you have to move out of the house. AC, noise suppression, fanning, racking, power upgrades to draw that kind of power. Ever think about a fire ?
You also missed the whole chapter on difficulty. If you can't grasp the aboveyou will never understand that.
If you project 2700 a month at today's difficulty expect to be paid on $2400 tops because difficulty isn't going to stop when you come online with 90gpu. Next month $1700-$2000. Next month $1500. But your bills stay the same, in 3 months you can't make rent. To keep up with 2700 you have to add another 10-15 GPU each month.