Karl Marx based his claim that all workers are exploited within
capitalism, upon the assumption that the value of a product is
basically determined by the amount of labour put into it.
More specifically, the value of a product equals the cost of raw
materials, usage of means of production (both factors are more or less
constant) and then extra value added to it because of labour (e.g.
coffee is worth more than the mere sum of water and coffee beans; the
extra value is created by grinding the beans, ...)
The point then is that a worker's wages are typically less than the
extra value of the product added by labour. Hence the worker is
performing some of the labour for free, hence exploitation.
Now my question: I can see the reasoning behind this, but I question
the assumption that the value of the product is determined by the
amount of labour put into it. For example, suppose one worker is
digging for gold, while another one is digging for stone. Let's say
the labour they do is roughly comparable, yet surely the value of the
product will be much higher in the first case?
Does this invalidate Marx' theory, meaning that workers are actually
not exploited? Probably not, but where do I go wrong?