I wasn't actually thinking that Miranda would go with selling her child. However, if you are going to the lengths of hiring an assassin it's not going to be that large a stretch to hiring a kidnapper and selling her off. And her mother as well, should she choose to interfere. Or, in either the assassin or kidnapping scenario, the PC could play "rescuer" to get on Gavin's good side.
Of course, if Gavin found out about it I am sure he would be outraged, but that's half the fun.
I'm just looking at it from the perspective of my evil PC, who's main motivation is to make as much money as she can to retrieve Imoen as soon as possible. Not that she cares about Imoen, but it's a personal insult that the cowled wizards took her. Besides, that Irenicus had some very interesting things to say...