Coman right now is worth 5-10 mill max. You're talking about a player who's never been a regular in a professional team. If he becomes good, it will also be because of the time and effort that team which nourished him put into him. That time could also be invested in other players, and is worth money.
28 mill for him is way too much, and was probably negotiated with the Vidal deal under the table.