rtw vanilla required each trader level building for the armor line. bi requires only the trader -- for good reason, since the other markets are not cost justified.

I really don't feel like doing the analysis over again, but I did one for rtw that, imo, applies just as much to bi. The upshot was that you'd be better off doing nothing with the money than building the market line, in virutally ALL provinces.

10%, remember, is only for the best trade provinces. And 10% means that for 10 turns you are loaning the game money. Moreover, with "inflation" that 10% is probably a real 5% (there is no "monetary" inflation in the game, but there is inflation in the sense that your income/expenses rise, so a given level of money generation is not worth nearly as much tomorrow as it is today)