Granted, she also has to live in Tokyo which is super expensive and then she won't be making this much money after graduation anymore - which might be in a few years
She also likely lives with her family (I don't think we have a single active member who doesn't) so that's probably not an issue. Most of the girls seem to live at home and come from families that don't seem to be struggling. The number who 100% take care of themselves is probably the minority. I think poorer girls tend to go for careers rather than idoldom.