Scott McTominay is worth whatever any other club willing to pay him. We should really take the bullet and sell him to the highest bidder, even if that's "only" 20m.
United current stature and performances are not really enhancing the price of any players we want to sell. Because United played putrid football below what's expected of them, the players value also reflect this. Arsenal and City are able to sell their squad players much higher is because they played good winning football. If Arsenal wanted to sell ESR for example, and interested clubs are seeing that well, he have to compete with Odegard/Saka/Martinelli, they'd think, he probably a good player but he can't get into the team when the players he compete with are better and playing better football. Therefore they can sell him for higher price.
But at United, when they wanted to sell McTominay, interested clubs will think, well, the players he compete with (Amrabat, Casemiro) aren't that good and they played horrible football. He probably an okay player but doubt are raised because he can't dislodged those who played bad and when he played he isn't playing that well.