Not sure, that is an interesting number. If they had 20,000 devs, with an average boot time of 10 min, at $5 a min (which seems high, if it's just salary), then it would cost them $1m. But given that developers don't work 24/7, and I also think that Google has less than 20k devs (the company is only 30k). But it could have to do with math outside of salary, such as power or maintenance costs. I think it was Google that did a study on how screen saver choice affected their power bill, so I wouldn't be surprised if they knew exactly how much a reboot cost.
You also have to consider the break in flow. A boot time of a minute, at the wrong time, can cost the mental context of the interrupted developers to get dumped on the floor. That costs a lot more than a minute to recover from.
