Burnwinter The fee is high due to his low wages would be my guess.
Maybe it just looks high because a big amount of add-ons is included, as speculated on Twitter.
It would be unusual, but since Chelsea need to raise money urgently maybe they agreed to something like this which allows them to put it down as a 50 million sale for now, when in reality the number they'll realistically get from us is much smaller.
We'll have to wait and see as the details emerge, but this would be my guess.