I think the fee is also really high because Leicester paid 30 million themselves, added to potential and age.
No self respecting football club sells key players at a small profit margin, it is usually at least double. If you buy a player for 30 million plus add ons and sell for 50, that isn't great business as you're not adequately compensated for losing a good player and replacing him with an unknown. The model has to give you sizeable profit to reimagine the squad.
The reason the fee looks massive is because Leicester already paid good money for potential, which they haven't had the time to fully unlock and now Chelsea pay a premium on that potential again.