If you actually watched Chelsea over the past two years, you would think Ziyech is the worst of the bunch. He is an awful player with a poor attitude.
Koulibaly improved as the season went on and has better recent pedigree. His wages are a problem, though.
Mendy makes a pretty insignificant amount. He won the CL two years ago. Why would Mendy have zero value? Huh?
1) I don't think Ziyech is a good player, but his WC revived some of his value and he's probably the one that you can most argue its been a Chelsea fit issue just as much a player issue. He's also probably the biggest name draw of the 3 players. I wouldn't pay a fee for him, but a small fee isn't out of the ordinary to me.
2) Koulibaly is on 300k/pw wages. No one is paying a fee for that. You likely wouldn't even get a team to take on his full wage in a loan.
3) Mendy may have won a CL 2 years ago, but he played ~10 games this year sitting behind Kepa, and didn't have a particularly good season. His wages aren't too bad, but they probably rule him out for a good amount of the bottom half clubs in France/Italy, and top clubs are not going to be coming after him. Could he garner a small fee? Sure, but not close to contributing much to the rumored combined fee.
Better question -- why did you think these players had little to zero value just 2 pages ago?