I don't understand.. do people not understand the basics of football?
AWB is 21. Zaha wasn't Palaces star players last season, AWB was. You're signing a RB for £50m who is British, who could potentially play at a top level for the next 10 years, and who could become one of the best in his position in the world. If you wanted to sell him in 5 years, you could probably get a profit based on transfer inflation, and if he keeps progressing (no guarentees but he looks like he has the tools).
Then you have Zaha, who is a player who's been playing since a young age, and who didn't make the cut when he moved to a big club. He's a risky player, who has potential to be a very good, but not world class, winger. Equally, he could only be suited to counter attacking teams not possession heavy teams which is the risk. He also wants to leave Palace, notably for Arsenal. He also has no resale value at all, because in 4-5 years, he will not be anywhere near as good as he is now. He has never produced remarkable numbers in terms of end product, and went off the boil towards the 2nd half of the season.
Wan Bissaka and Zaha are nowhere near comparable in terms of expected fees, it's a silly argument and comparison to make.
I think £40m is a fair opening offer. You never open negotiations with your final bid, and to be honest, he isn't worth more than £60m considering everything said above.
The final offer will most likely be £35-40m and Chambers (and a player on loan/nketiah/smith rowe), which in theory could mean only about £9-10m goes to United.