First off is it 25% of the gross transfer amount or is it 25% off the net amount? I'd think it's net.
If they sell him for 100, they would net 63.75, for an asset they spend 15 acquiring, so 425% annual ROI. By any accounting / economic measure that's more than fair. Of course they can ask for 150m, they have the right, doesn't make it reasonable though.
Dembele is a talented guy with 1 year on his CV. We've seen BVB players look world class in Bundesliga not being able to maintain the same level abroad (Kagawa, Mkhitaryan, Sahin, Gundogan etc).
You tell me 80+20 is ridiculous, I tell you it's more than double the world record transfer in Germany. Neymar has really distorted the market.