These assumptions almost certainly make the army appear much cheaper than it is, but even so, that's 400,000g. How much of that money comes from trade taxes and tariffs as compared to property taxes, poll taxes, taxes on the nobility, and so forth? Difficult to say. But given the scale of the funds being worked with here I'd say that it's a pretty sure bet that Mathilde, entitled to 36% of the profits from an economic juggernaut whose scale can almost certainly be reasonably measured by the standard "what fraction of the army's maintenance cost does it make in profits", will be making thousands of gold annually. If the EIC is making 1% of the army's maintenance cost Mathilde would still be making over a thousand a year.
That debt will be gone within a handful of turns at most and then we'll be making as much as we did from Asarnil's book every six months. We're going to have to get this sorted with the Colleges and we will not be able to continue owning this much personally without going into some kind of special arrangement regarding it, which will probably require either us or someone else from the College to be actively overseeing this money and making sure it is directly and unquestionably poured into projects which are in the interest of the Empire.