The true value of a bitcoin will be less than the total amount of productive energy that was diverted to its production if this whole exercise turns out to have been a mistake.
What if the math behind Bitcoin is flawed, and tomorrow it is announced that anyone can steal bitcoins from anyone else, and there's nothing we can do about it. You agree at that point the "true value" of bitcoin will be nearly zero, right?
(I don't mean to imply that there's much of a chance of this, but it's possible.)
The total amount of productive energy that was diverted to the production of bitcoin is roughly a lower limit on the total value that miners expected bitcoin to have. But the miners might have been wrong. (I say roughly because some miners in the past and probably even today are mining for the fun of it rather than because they think they're producing something valuable. There are also miners in the past and probably even today that are stealing electricity and processing power from others.)
Good point, in that case a risk factor should be added as well to the calculation, I do believe my "borrowed" corporate finance textbook has examples of risky endeavours, however bitcoin's risk is sort of unknown at this point and I could probably only create a time based speculative risk factor (we've existed for this long without any fundamental flaws, the chance of a major flaw decreases by n% every year.) I think that risk assumption is valid, particularly because we have very excellent programmers/white hats that have been attacking the bitcoin protocol since its inception, and it's been adapted and reinforced considerably since.
There used to be a ton of hobbyist miners, which is why I believe the price before used to be quite low in comparison to now. There is very little chance of "hobbyist" miners being able to produce any amount of bitcoins, compared to the "serious" miners who have invested considerable amount of economic value (aka diverted labour) into buying bitcoin ASIC mining farms, which is a partial reason why I believe the price increased, the miners themselves value Bitcoin more now than they did previously. Also the amount of money going to pay electricity per PH has been decreasing steadily, which means that the people with "free" power don't have anywhere near the advantage they once had.
Again this all looks past the speculative nature and looks just at the concrete value of a single bitcoin, which I think will continuously rise until the "value" reaches a meaningless value (in relation to Fiat, the "current" standard of potential labour)