Check out the last part of my recent post:
https://steemit.com/tether/@steemboat-steve/tether-bomb

In the case of Bitcoin (and all PoW) it is also the cost of actually creating the blockchain as well.

People mined gold at a loss, much in the same way as some mine BTC on inefficient rigs, they believe it will appreciate in future.

There is also the capital costs involved, not just the electricity to calculate the works. If Nvidia's recent stock rise is anything to go by then this cost is pretty damn big