M8 I'll give you a run-down. If you look at how things are actually produced in society it all comes down to labor. All the costs are related to labor at the end of the day. If you invest in new machines those have to be built; to build the machines the raw materials have to be mined, shipped, and refined; and once you've acquired the machines you'll need more labor to operate and maintain them.
How do you figure out the value of the machines before they can be sold? Not from any intrinsic properties or even their use-value – what they're actually meant to do, because that has little to do with their exchange-value – what you actually pay to acquire them. You figure it out from the production process – from labor.
Modern relations of production grew out of an economy measured by work hours, and it remains so today. Whatever is produced in society is measured by the labor required to produce it. Keep in mind that labor itself is commoditized as labor-power and sold on the market. Wages are the exchange-value of labor, and exchange-values have little to do with the quality of the commodity itself, its use-value, or other subjectivities. Skilled labor is naturally worth more, but that is because it can produce faster, better, than unskilled labor – it is a quantitative measurement rather than qualitative.
The exchange-value of a commodity, therefore, is the socially necessary labor required to produce it. All production is collective and thus social – hence socially necessary. It's not a matter of individual productivity. It's about the average productivity required to produce a quantity of commodities. Some workers work harder than others, or are more skilled than others – but not all are skilled or motivated. Wages and salaries are not set according to individual productivity but the average. The production of a commodity will probably require all kinds of different labor, but that has been purchased as labor-power. Now it is a matter of what can be produced in the time the workers labor – their quantitative output.
Once a given quantity of commodities have been produced it is a matter of selling them – the value invested and contained in them cannot be realized until they have been sold. This is very important for the capitalist. Knowing the cost of production – the labor required to produce the good or service – the capitalist sets an exchange-value according to market conditions and competitiveness. If the goods sell, the investment is returned and then some in the form of surplus labor value. If not, there's trouble. But we'll focus on the former.
As the workers produce collectively and sell their labor-power in exchange for a wage, they produce more than what they receive in their wages. Why else would the capitalist purchase labor? If the capitalist's only return was the cost of production, there would be little point. The capitalist buys in order to sell: M–C–M'. In other words, they use their money to buy commoditized labor-power and then sell the good or service above the costs of production, realizing a profit – surplus labor value, or, more often, surplus value. That surplus value, apart from what is used for their own consumption, is then reinvested or handed out as dividends by the capitalist – thus capital accumulates by its own logic playing out in real production.
Bourgeois economics denies this root and branch. It concentrates on things like "human nature" and other subjectivities which ignore how things are actually produced. True to the workings of capitalism, things like property have a life of their own in bourgeois economic theory, while living beings like workers act as the adjutant to the production process, not its driving force. Bourgeois economics inverts reality, placing property above people – and thus social relations of production.