This is an interesting one.
Zaha is probably a £40m player. But Palace's survival almost entirely depends on him staying so he's worth £100m+ to them. On top of that 20% of the fee goes to United so Palace really need to sell him for a massive fee.
Then there's talk about gentleman's agreements, letting him leave, souring a relationship that's been so great for 5 years. He's 27 so this is realistically his last chance.
I think this one will end up in player strikes/angry chairmen venting in the press etc.
He'd be a really good signing for Arsenal.