Reportedly the trade_base_bonus increases the trade value of trade resources in a given province by a certain factor (5?). Therefore the effect is dependent of how many such resources in a province are present and how are they traded with other provinces. Thus for a single province province faction surrounded by enemies and enjoying no trade at all the effect is next to none, while a resource-rich province with many trade connections to friendly provinces nearby the effect can be huge.
Treat with care - it is powerful, but quite realistic mechanics and something the CA actually managed to get pretty well!
