"The board is designed for a maximum input power consumption of 300 W"

So one hour at home:

Power used: 300 Wh

Price: $0.20/kWh

Total cost: $0.06/hour

Google cost: $0.70

You could say it's 10X more expensive, but you need to include all additional costs, power is just a fraction of it. The GPU itself is $5k, which over - let's say - 3 years would cost $5/day, or $0.20/hour, and if you're using it a third of the time, just that amounts to $0.60/hour. Add everything else (you might use less than 1/3 of the time) and it would be far more expensive than using it in the cloud. As it is expected to be.