As long as the player isn’t too old, transfer fees go in both directions. Pay 30m, 4m salary, sell after two years and let’s say for argument only get 23m back. Net cost 15m.
Tarkowski 6m salary for four or five years adds up to more.
It’s important not to let salaries go through the roof. A big wage bill for a team that’s not yet performing means players can’t be shifted or even sent on loan sometimes. By paying transfer fees, as long as it’s not for players older than 28-29, you get the option of moving them on if needed.