I'm still thinking my theory that she could be ill and in need of urgent and expensive treatment is the cause behind Maya's need of money fast, as that could be the difference between life and death. But it's just one possible scenario, and at this point there isn't any kind of evidence to support my claim, it's just intuition. Every other theory could still be valid as long as it doesn't contradict the known facts.
The weakness in this theory is that it doesn't jive with the conversation with Maya at the end of C.3 IMO.
Whatever Derek told her, it confirmed something she's been thinking since the beginning (not to trust dad). It's
not new news of a sudden urgent need (whatever the cause). Further, after MC asks her what it was, why would she be afraid of how we'd react if what she was going to say was "my mother is ill."
The bigger clue though is this:
The reason she wants the money is the same as before. She was going to give up on it due to how onerous the scavenger hunt was, but is she is even more resolved now - partially because of what Derek just told her, and partially because of (the support and the growing relationship) with the MC. If it's for mom's sudden illness, you the MC would obviously have nothing to do with her realization that she wants the money or making the reason bigger, and it would be bizarre for her to structure the last sentence in such a way IMO.