The only reason big farms are profitable because they have profitable conditions.  You could equally have a small amount of miners, or just one, in good conditions and it would still be profitable.

For example, you live in a place with low electricity costs, you buy an Antminer or Avalon miner and put it in your basement/garage, then boom you're mining and it's profitable.  Often it'll still be more profitable than most investments which we forget tend to actually only get ~10-20% per year for people, and this is riskier but it's not so risky that it makes it unreasonable.  Cooling systems etc don't necessarily need to be very good or even in place at all unless you're already doing a big operation.