@Tersione—
Takao could conduct a series of scientific experiments, by wishing for different sums of money and both assessing the value of what breaks and the number of stamps that Nazuna gets each time. He should probably start with ¥100, increase this to ¥500, then to ¥2500, &c, stopping if there were strong indications that wishing for money resulted in particularly bad outcomes. It is not
inconceivable that he would infer that the very best strategy would be to wish for ¥1 many times, elevating Nazuna with a minimal material loss to himself.
Of course, an approach of this sort is highly unlikely in a comedy manga.