This is not what Marx believed, he painfully toils through literally thousands of pages arguing for his theory of value, and is not that there is one objective value, which translates to the exploitation of the worker. In fact, his theory is grounded in the profound complication of what we consider 'value', specifically its duality under capitalism: "use value" and "exchange value". What you are most likely referring to is " surplus value" which is something like the amount you can point to that comes from the exploitation of the worker, but it comes from a very complicated pseudo algebra of his own design.

The best thing to pick at, if you are arguing with Marxists, is the general "labor theory of value" and whether that ultimately is correct. The labor theory of value has to do with how we assign economic value to things on the market, and that it ultimately is from the labor of those who produce the product.