Hard to say when every team's most promising youth prospect could be different levels. For Arsenal, Nwaneri is supposed to be one of the most promising youth prospects in all of England, but I somehow doubt we'd get a 20M fee for him. Maybe that's our poor track record for selling that has me feeling that way.
Bellingham went for 30M after starting 30+ games in the championship and having several clubs chasing him. That's a single data point, but I think everyone here had heard about Bellingham to a degree before he went to Dortmund. Whereas, I didn't know who Omari Kellyman was at all until he was sold for 20M to comply with PSR, and I think I have a decent grasp on some of the top prospects.