Look at it another way - Declan Rice will go for £100m. Fulham will be looking for well over £50m for Joao Paulinha if he moves this summer.
The Rodri fee was a few years ago and now looks like a bargain.
The comparison seems out-of-date now.
I don't know exactly what sort of figure I'd be putting on Onana because you're paying for potential, but he does look like he has an awful lot going for him if he can harness it all. If we paid £26m and owe 20% of the profit to Lille, a fee of £50m gives you a profit of just over £19m. All depends what sort of premium you'd be happy with after only having him 12 months. Personally if we could double our money on him (which would mean selling him for about £60m after allowing for the 20% sell on clause), I think that's in the region of what I'd want based on him having 4 years left on his deal, his age, the obvious potential there and the Declan Rice valuation.