You would not get a billion dollars from that. Someone else would see your mill and make their own mill. A thousand other people, in fact, if there was a billion to be made, so each would get a million, or less. The intense price competition would also bring your profits to near zero.
But if I were first and built them better, continually being at the forefront of mill construction, it might be enough to make 0.001% of my nation’s GDP for a few years.
In the US that would make me a billionaire.