>>28041
There is not just one value, there are others. There is a relative value which based on different factors, there is use value which depends on the utility of the thing which is part of it's relative value, and there is exchange value of course and others I probably didn't know about.
But every item, goods, commodity and service have something common in them (a common denominator if you will) which can be used to determine their value which independent from any perceived value - which depends on the subjective judgement of the people - they have. It's the labour behind them to produce one unit of them (well at least Marx called it labour, I would say energy since labour can be determined in joules or spent calories or whatever, basically units of energy).
Why diamond is more expensive than coal? Their rarity their utility, their percieved value will decide how much it will cost to someone. But beside that we can calculate that 1 unit of diamond is more labour intensive to produce than 1 unit of coal. Let's say I want to produce 1 cubic meter of both. I have to spend way more labour on producing this 1 cubic meter of diamond than coal (it is related to it's rarity ofc, but rarity in itself can be perceived relatively), hence their value in labour (can be also expressed in working hours, but I would still remain at spent energy) will be different. This "labour value" doesn't depend on any relative or subjective factors, hence this is an objective way of expressing value.
I think this can be understood with common sense.

Btw capitalism is a failed system. The Great Depression shows it's failure, it had to be saved everywhere by state intervention and since then it kept afloat everywhere on the world with governmental regulations. The only ongoing debate on this is how much regulations it needs.