If you look at 2019/20 and 2020/21 combined , Mina was available for ~70% of PL games.
This season that has dipped and it's fair to say there are Q marks on his durability. But teams will take a risk on him. Getting Mina for 70% of PL games probably improves at least half the teams at CB in the PL. IMO he starts for 12-14 teams in the league at CB. That's why he'll command a fee and someone will take a chance on him if he's available for ~20M
Not a chance Everton get close to £20M after the season he's had. There are more than question marks about his durability - it's a clear fact he is injury prone. He's managed 90 minutes ONCE since October last year in ALL competitions. That's far more relevant than averaging out PL appearances only (including bench cameos) from 2-3 years ago.
And he isn't someone who improves 12-14 sides without question. Even before his recurring thigh issues.
Nobody would come close to matching his wages at Everton with those question marks. And he isn't going to go elsewhere for less without being compensated at least in part for that loss of earnings. That money comes at the expense of the transfer fee - either the money a club is willing to pay goes on paying up the contract or Everton reduce the fee massively to enable the buyer to up their offer to the player.
Or even Everton agree to part finance his wage until the date his Everton deal was due to expire - plenty of precedent with the likes of Leeds and Sunderland how players wages kill their transfer value. Or even the likes of Rodriguez - instead of paying Real Madrid we took over the wages at the expense of a transfer sum. Same with Rooneys "£7M" deal to MLS - that 7M wasn't going in Everton's coffers - it was paying up his deal and getting him off the books.
How you think Everton end up walking away with £20M in those circumstances is a leap for me.