Lol, she let him bang her for thirty bucks. Let's face it, the way this story is going she just needed an excuse to let him bang her and so gave a dirty cheap price you can't even buy a new video game for.
It's a pornhwa, the goal is to titillate, not to be realistic.
IRL She would tell him to F off, banging is for regulars only. No reason to break the law on some johnny stranger in for his first time at the shop.
And you're right that most are in it for the money, but there are often many women who are manipulated by their "loving", deadbeat pimp of a boyfriend.
Some do it for their family (e.g. single mom, ill parents), some do it for drugs, some do it for the expensive Instagram lifestyle, some do it for cosmetic surgery addiction (common in JP/ROK), some do it because of sexual addiction, some do it because of other mental illness/disability which makes her unemployable otherwise, and some do it to get ahead in life (e.g. buy a house, get an education, have financial freedom, even emigrate).