Having the read the article, I understand that the market value is a crowd estimation? As in, internet users can say how much they think a player is worth? With zero statistical analysis or requirement to have ever watched the players?
That's where my issue lies. Have you ever watched Nouri play? The "value" is a fabricated number, as I said at the start. You can not rate transfers based on the suggested value. Do some research on the player, watch them play and form your own opinion.
On Mendy, thats how transfer negotiations happen. Start low and meet at a price both teams are happy with. How do you know what role they intended for Mendy? Kepa is awful, so they went to get a replacement. They're stuck with him, but NFL analogies do not apply to Football. I doubt you or I could name 50 better keepers, you can try though, if you wish.