I have heard a rumour, and I stress a rumour, that the main fee will be around 20 mil with add ons amounting closer to 30, albeit the add ons are unlikely to happen (such as winning the league).
Basically Palace want it to look like they are getting a lot because he is a fan fave and they don't want to risk the backlash of supposedly selling one of their best players for a reasonable fee.
The more I think about this the more positive I get. He would be the best winger we have and probably the most consistent too.