Almost all of the income results from ports. Check the settlement details screen before and after adding a market. Even in the absolute best trade provinces, the amounts being added are trivial (and usually do not justify even the lowest level market building -- the trader)

10% per annum return is decent in real life; in a game world, where financial demands are, ahem, a BIT more stringent and urgent, it is simply unacceptable. (especially when the alternatives -- e.g. spending money on troops for conquest -- are so much better)