Well, how about if she and her john agreed to $100, and he only paid $50? Can she sue him? She's gotten herself into a situation too goofy for anyone else to competently referee. After all, if the law gets involved, what principle is it defending? The sanctity of contracts? There is no enforceable contract here, because prostitution is (in America) illegal. How about protection against illicit sexual congress itself? She's in the business, and solicits violations against legally sanctioned boundaries so routinely, it's not feasible for the state to police each one.
Briefly, she's toying with the idea of law, and given the shortness of life and the fact that more reasonable people have matters that merit the courts' attention, I don't see the state as obligated to join her game. If someone shot her, that would be another matter. But in this case, she should just be arrested for prostitution, period. I don't even see the "theft" argument.
The woman legitimately has remedies in this situation, but they are those within herself, such as reforming her life.
There is no general, blanket presumption of consent. Even a married woman, who has an ongoing legally-recognized sexual relationship with her busband, can prosecute for rape if she can prove force, threat of force, or explicit coercion (overriding her right to consent or refuse.) Hard to prove perhaps, rarely leading to conviction perhaps, but she still has standing to bring the charges.