Yeah you don't...
None of those came from Everton mind you. It is different buying a player in the Belgian or Dutch competition than from a PL team. Démbélé came from Fullham and Tottenham had to pay his release clause of £15m, if we had acted smart with Fellaini we could have also signed him for his release clause a month earlier for £21.5M. Its a bit more, but Fellaini is considered a bigger player than Démbélé and Everton is no Fullham.
If you want to compare, compare apples to apples and pears to pears.
De Bruyne made his transfer from Chelsea (and hardly playing there) to Wolfburg for £18m
Benteke was a nobody when Villa bought him from Genk, Lukaku was unproven and very raw when Chelsea snapped him up at Anderlecht. I have seen plenty of these players in the Belgian competition and I never thought they were going to make it the way they did and I thought at the time those clubs overpaid. But they took a gamble on them and it paid off. Those transfers are not comparable to Fellaini at all.
If you want to sign Benteke today you'll also pay upwards of £20, same with Lukaku.
Another example is Kompany, he cost City £6m, but now you would have to pay the world to get him.
None of those players when they made their move from Genk or Anderlecht to the PL were established internationals aswell. If you want to judge the transfer of fellaini judge it in the right way, his transfer price makes alot more sense then. Not that he is worth £27.5m, but £21.5m would have made sense for him (probably a bit too much, but comparable to for example the £18m that was paid for De Bruyne).