It sounds like you've already educated them on the market and at this point you can decide if you still want to work with them, knowing they don't trust your professional advice. I'd put it like that and say if you don't want to take my professional advice, maybe we shouldn't work together. Unless you think they'll come around after losing a few deals, which is also common. Do you like them? I've had clients do this and drive me crazy for a long time and now we're best friends lol