Well, if you are correct in your assessment then he has to be worth circa £10m. No-one will pay anything close to that. So I hope he stays. Regarding his career, he didn't really have one when Chelsea released him and the likes of Palace rejected him so one would like to think that he feels some gratitude towards Gus and the club. He is only 23 so it's not like he is in the last chance saloon for that big money move.
So what makes Zaha worth over £10M? He is no better and no more valuable than Bridcutt in my view.