Usually the guild masters heads up works. The amount of money that the resource can make always changes, just like he says. I have a mill in Dhirim, a iron works in suno, and a wool thing in Praven. At one point i was losing money at the iron works, even though I was used to making over 900$ a week.
So it all depends on how you would like to go about it. The Bread mill is super great at feeding your armies, I wuv it.