But if I import timber for 5$, then my lumber mill manufactures it to lumber (WM price 15$ - price of used timber 5$ = 10$ profit for me) and then I let manufacture this lumber to paper (WM price of paper 50$ - cost price of lumber 5$ (since i provide it myself, i have only to pay the timber) = 45$ profit). This would be the way in real life(ok, ok, i would have to pay my workers and taxes, and transport etc.

) .
I don't understand how the machanics work in the game. Does the lumber mill sell it's lumber to the WM first so the paper mill has to pay the full price for lumber? It would make sense just adding the profits of all factories then...