Foucault argues his case by arguing that it was Ricardo, not Marx, who freed labour(or labour power) from being secondary to monetary and other kinds of exchange. Labour power was the true “measure of value”; not products or their monetary values (as measured in other ways). Goods - all goods - gained value because of the labour power expended upon them; not because of "scarcity", "intrinsic value", or anything else. Labour was at the very heart of economics and thus the determiner of value and exchange.
That is, if the capitalist sold goods at the value of their (real) labour power (or the units of labour involved in production), the he wouldn't make a profit because his workers would be paid at the correct (or fair) price for their labour; as expressed in the "labour theory of value". Thus the capitalist must demand extra labour from his workers - over and above what would generate the actual price of their products - to guarantee a profit. That extra surplus value of labour is what generates his profits and which, similarly, is taken away from the workers themselves (who won't now receive a correct or fair price for their labour).
Even if the theory can be understood clearly, it still involves incredible metaphysical assumptions.
Who's to say there is even such a thing as a "fair value" let alone a "correct value" of given labour? How can an economist decide what belongs to the labourer and what belongs to the capitalist? The facts can’t decide this matter. This transference of value from labour is either metaphorical or metaphysical (or both).
Without arbitrary stipulations, societal customs, norms, economic traditions, etc. none of Marx’s theory is at all factual. It is metaphysical. In a sense it's not even theoretical. It’s as if “real value” or “true price” etc. existed in the ether before any system of capitalist economics even began. Before and during capitalism Marx saw it that there is an absolute value of labour and therefore an absolute value of produce. And if these things are indeed absolute/metaphysical, then surplus value will be an absolute too.
It must be an economic fact that the capitalist genuinely creams off surplus value from his workers.
It must be an economic fact that there is a given price for products given a certain amount of labour.
But how could there be? These aren’t facts. Thus they must be metaphysical or even ethical/political impositions on the economic facts or realities.
This is like the distinction between "fact and value". Marx imposed moral/political values (literally and metaphorically) on certain economic facts and realities. But that imposition isn't itself factual. It's metaphysical and moral/political.
All that Marx offers us is are basically metaphysical assumptions about value which are themselves determined by Marx’s hidden normative/moral position on the status of the worker vis-à-vis the capitalist who employs him.
Thus Marx goes way beyond economics. He ventures into the realm of metaphysics; as well as into the realm of normative/moral economics.