For purposes of immersion and realism, I've come to see the very high level 3 advisor "salary" as representing the funding for all of the many expensive projects that that advisor is suggesting/undertaking. In da Vinci's case, that +3 per-month to administration represents all sorts of expensive projects (not to mention the +10% to production).
If the 226 ducats hiring fee and the 14.61 monthly cost were actually his salary, then he would be individually wealthier than many countries! ...including my own, which is why I can't pay him.
So if I keep my army small to pay a level 3 advisor and, as a result, an enemy takes advantage of the situation and attacks -it's not because I'm building a mansion the size of a province for my advisor; rather it's that I'm saying yes to all of his brilliant (and, of course, very expensive) ideas and projects.
I know that gameplay and balance are more important than realism, but immersion and realism are still nice to have; even as abstractions.