The difference between the price of bitcoin versus the price of bitcoin as it would be if it was only used as a medium of exchange rather than a speculative asset.
The bitcoin network adjusts the block difficulty (effectively the energy required to mine 1 BTC) every 2016 blocks such that the rate of creating blocks is fixed at one block (currently 12.5 BTC) every ten minutes.
If the dollar value of energy required to mine 1 BTC is significantly less than the current dollar value of a bitcoin then it is likely more hash power will be added to the network by the bitcoin farms, further increasing the network's hash rate, and as a consequence the block difficulty and therefore the price of the energy required to mine 1 BTC.
In general things are only as valuable as the "market" is willing to pay for them.
Sure, I was agreeing / checking my understanding was correct :)
Let it rest for a while and I believe it will have more races up-and-down the 10k-22k range so people can enjoy their gambling ;)