I'm researching the source of profit. In Marx I've found the following passage:
If then, speaking broadly, and embracing somewhat longer periods, all descriptions of commodities sell at their respective values, it is nonsense to suppose that profit, not in individual cases; but that the constant and usual profits of different trades spring from the prices of commodities, or selling them at a price over and above their value. The absurdity of this notion becomes evident if it is generalized. What a man would constantly win as a seller he would constantly lose as a purchaser. It would not do to say that there are men who are buyers without being sellers, or consumers without being producers. What these people pay to the producers, they must first get from them for nothing. If a man first takes your money and afterwards returns that money in buying your commodities, you will never enrich yourselves by selling your commodities too dear to that same man. This sort of transaction might diminish a loss, but would never help in realizing a profit.
Karl Marx - "Value, Price and Profit", 1865, the last pararaph in section VI "Value and labour"
I can't construct a numerical example which could illustrate this logic for me. It seems like even critics of Marx didn't argue against this passage just like it was very clear to them. But it not clear for me.
Please check my logic. Consider a capitalist who has costs of 10 USD per unit including others' margins of 5 USD. According to Marx she has to sell for at least 10+5=15 USD and it will be just a zero profit. But mathematically 15-10=5 USD and it's not zero. May be Marx means that these 5 USD of profit will be unavoidably spent for a bare survival of the capitalist. But why, if so?