depends how you assess value. too many times, people line up players "as all things being equal" without taking into consideration key factors namely:
1. how old the player is
2. potential resale value
3. marketability
4. whether the player wants to go/the club want to sell (e.g. D. Costa and Courtois)
5. how long their existing contract is (Hazard may be touted at £86m and Zaha £80m, but one can leave for free within 12 months and the other has 4 years left on their contract)
6. the value and contribution to that club. For example 17/18 season, without Zaha it can be easily argued Palace would have went down. Don't have the stats but their win % was shite when he was injured, plus his goals/assists versus the team total is ridiculous. taking the fact that he may be the reason for palace staying up (and relegation is worth how much?), there's a reason why palace value him as such - doesn't mean they'll be able to sell him for that price (MV solid £50-60m)
7. Apparently United have a sell-on clause worth £15m. So if Zaha sold for £80m then it's £65m Palace, £15m United