To the value is subjective so it is okay that the unit of value is subjective crowd.
Value is subjective to the individual but objective to mankind.. I can say I dont value water because I am not thirsty, I can never say that mankind doesnt value water.
Further the measure of the subjective nature of value is called price!!!!
This happens in other systems of measurement too.
Imagine you weight a bowling ball on earth and it weighs 10 pounds then you weigh the same ball on the moon and it is 2 pounds. The pound didnt change, human perception changed. We could say that the ball weighed 10 earth pounds or 10 moon pounds but we dont. We represent the change in perception with a qualifier. The ball weighs 10 pounds on earth, earth being the qualifier or the ball weighs 2 pounds on the moon, the moon is the qualifier.
We could do the same thing with currency we could say water is worth 10 units of desert currency or it is worth 1 unit of beach currency, but we dont.
We use price as the qualifier to represent the change in human perspective!
People say that gold and silver are not a constant because their value measured against other items changes. It is not the UNIT that changes it is human perspective.