If you are using over a million watts (an industrial user), the
price varies throughout the day between $8/MWh and $850/MWh (with an average of about $50/MWh).
The traditional model is to run your miners 24/7 until they no longer pay for power. However, if you want to run the miner intermittently (either for heating or load-shedding applications), it may be possible to get obsolete hash-power at a cheaper rate.
From the
[HAVELOCK] DataTank Mining: 1.2MW 3M Novec Immersion Cooled 2PH Mining Container thread, I learned that Bitcoin mining is pushing computing density limits. That said, for the high density they describe, you need to remove heatsinks and plug the boards into some kind of back-plane (that not all miners support). For such industrial mining, it is probably cheaper to buy current-gen chips in bulk, and make your own boards.