I'm pretty sure, that by this logic, if you combine all the "$15/month" things that someone should have money for, they'll eventually be left without any of that money. Every goddamn website wants $15 per month, and even after they get it, they still show you ads. Even youtube, if nothing else, there are the in-video ads ("this video is sponsored by,...") that an adblock skips.
This person likely can afford it and some others without any noticeable impact, I just don't find it appropriate in that circumstance. If you get $15 worth, and can afford it, pay $15, plain and simple.
