Before we build a manufactory in Oltenia vs after we build a manufactory in Oltenia.
Trade value went from 0.32 to 0.87
Production value went from 0.67 to 1.79
That is a pretty huge double dip. We profit from both the production of the good as well as the trading value of the good.

Long story short: Manufactories provide more value than they say. You should build them.