@Akera93 I'm guessing since she's poor she thought he would bring her in as a servant or something. She probably thought a duke wouldn't marry as commoner.
But she sold herself for an extremely high price if I got that right so I don't think she sold herself to be a servant, that doesn't make sense. She doesn't seem like the type to be proposing to "sell herself" in the vulgar sense either....She also didn't advertise herself as a great magician . I don't know what she did. She didn't show anything special about herself and yet went there and dared to propose a high price for something she isn't specifying (?)
@Akera93 she showed her purifying powers (that spark that happened when she touched him last chapter) which is extremely rare and I think she mentioned being the only one currently alive with that power, at least according to what she remembers from the book (which is also why they both said that the price she was asking for was actually a bargain). Her "selling herself" was mostly meant as her being his exclusive purifier, at least that's the way I saw it