I will quote myself ... "...nothing happens when I add a trade module to that starbase", so, yes, I added the module to the starbase. I've reread the manual, and the initially 5 bonus is given for the resource starbases only. But still, what about the trade module bonus ? I know that there is a trade bug in ToaT, which makes all trade routes give the same amount of cash no matter the distance between the trading points, but I don't think it affects the bonus from a trade module, I think ...