Obviously I don't know what your finances are like, but is it possible she's just enjoying herself and considers it a hobby? Comparing it to other games, $100/month can seem ridiculous, but comparing it to other hobbies, it might not be that bad.
I used to be unwilling to spend any amount on a mobile game until I thought about how much I used to spend playing Magic: the Gathering. Sometimes hobbies cost money.
Have you tried saying, "Please don't ask me that anymore"?
That will address the exact problem without being rude, without offending him, and without opening it up for more questions. You don't owe him an explanation, so don't leave an opening for one. Just say: "Please don't ask me that anymore."
If he asks why, you say, "Doesn't matter. Please don't ask me that anymore."
If he offers an explanation for why he's asking you that, you say, "Ok. Please don't ask me that anymore."
Neat and easy. No unintended consequences.