Quote from: Hermonminer on April 15, 2014, 07:43:22 PM

A mining rig is a tool used to achieve some other purpose which is to mine BTC.

I believe the term everyone is looking for is "sunk cost".

Quote from: Wikipedia

In economics and business decision-making, a sunk cost is a retrospective (past) cost that has already been incurred and cannot be recovered.

See: http://en.wikipedia.org/wiki/Sunk_costs - especially, the section that describes decision-making based on costs.

Quote from: More Wikipedia

In traditional microeconomic theory, only prospective (future) costs are relevant to an investment decision. Traditional economics proposes that economic actors should not let sunk costs influence their decisions. Doing so would not be rationally assessing a decision exclusively on its own merits.
...
Sunk costs should not affect the rational decision-maker's best choice. However, until a decision-maker irreversibly commits resources, the prospective cost is an avoidable future cost and is properly included in any decision-making processes.

The cost of your mining hardware is a sunk cost. In a rational economic decision, that cost is immaterial, it's already been lost and can't be recovered in any way, which is especially true in the world of mining hardware that becomes "obsolete" so fast.