[ajross]

> There is a reason the market price of old mining rigs is not 0.

Electrical costs vary by a factor of two or three around the world. Maybe five at the extreme outliers. Modern mining hardware is tens of thousands of times more efficient than the stuff used less than a decade ago. Also due to growth in the market, the rate of production has been increasing rapidly so the total number of old rigs is negligible anyway.[1]

To rather accurate first approximation, all BTC mining is done with hardware less than two years old or so.

[1] Also also, the recent crash has had the effect of depressing mining revenue and accelerating the retirement of the old junk.