First time I've ever seen anyone use Transfermarkt as a reliable yardstick for player value. Mendy seems like the only player there that you'd be confident of receiving a fee for in a normal market
It's not 'reliable' by any means but provides a ballpark figure. When a player does well the estimation goes up, when they play like shit it goes down.
The club have paid a total of €115M for Mendy, Koulibaly, Ziyech and Aubameyang which was deemed 'fair market value' not so long ago. Apart from Mendy's first season at the club they've all been disappointing signings so it's no surprise the values have gone down significantly but for the most part they're by no means useless players even if they're not good enough for us?
For Mendy I'm 100% sure we could find a buyer from a European club at a similar value to whatever the Saudis will pay for him, and his wages aren't big either (£50-60K/wk) for most clubs in the European top leagues. Here it's probably a case of the player himself fancying a big pay day, having been a late bloomer and still on modest wages even after reaching the top.
With Ziyech it was just a few months ago we were a delayed fax away from shipping him off to PSG, albeit only on a loan deal with a buy option, but still it says a lot that a club of PSG's level were seriously interested in him to begin with and really pissed the deal went sideways. I'm having a really hard time believing there wouldn't be any kind of market for him if not for the saudis.
For Aubameyang there have been numerous top clubs after him all year (Barca, Milan, Atletico etc.) and there was an offer from MLS too so there's definitely a market for him as well. If we were to grant him a free transfer there would be clubs lining up to take him off our hands so is it totally out of the question there could be someone willing to pay a €4-5M fee for him too? For a free transfer the competition for the player's signature would probably mean higher wages anyway so paying a small fee would be a good way for someone to knock out the competition.
The only one who'd be really problematic to shift is Koulibaly. To break even with him would demand approximately a €25-30M fee. The fee itself probably isn't that obscene for a lot of clubs but the biggest obstacle here is the wages.