Why are you so certain about this though? Were you so apprehensive about the other midfielders we bought too? I'd say Arthur has proven more and shown more potential upside now by a longshot than either of Gomes, Paulinho, and arguably Denis did before their transfers. Arda was much more proven but there was very little upside to his transfer given his age and everything. All in all, I don't see why Arthur is treated with this asterisk next to his name if we didn't give that same doubt to everyone else we bought.
The only way it makes sense is if you think of everyone from Brazil, even the very best players, to be very low probability of success. Because Arthur having succeeded at the Libertadores level is showing he's quite possibly the most proven talent to buy from Brazil since Neymar, arguably even moreso than Gabriel Jesus (though I'm hazy on when City finalized his purchase relative to the Olympics, which would change things). He's won the most prestigious SA trophy, MOTM of the final, team of the season, and gets rave reviews from everyone who watches him; does he still warrant being seen as someone that's more likely to flop than not??
That being said I agree with 150m for Coutinho > 50m for Arthur. I just think Arthur's chances of being a success are pretty solid given the context.