Onana does look like he would worth about 40 max 50 million to be fair, I do not think that the offer is too bad. What are they expecting for Onana? 100 million? Lol, that's the amount of money you could expect for a decent goalkeeper these days and not a lot more and even all over online, the worth for him is about 35 to 40 million range so the offer is not bad at all.
Onana is already not too young, 27 years old and $100M is too high for him but football transfer market is inflated recent years. 20 years ago, if you pay very high fees, those players you bought would be super star players and already won Ballon d'Or like Kaka, Shevchenko, Zidane, Figo. Nowadays, with $100M, Chelsea bought Mudryk when they player has yet made his outstanding performance in big leagues and Champions League. Like Manchester United spent $80M to buy Maguire, unbelievable so I am unsure about the cost for Onana.
I understand Inter doesn't need to sell, they can just keep him, and when you are not willing to sell then it becomes a lot harder to buy that player for anyone else. However, it looks obvious that we are going to end up with a situation where they are going to provide some more money. I would say offer 50 million straight up, and do not offer a dime more, he won't worth more than that at all.
They don't have too wealthy finance and if any club bid high price, they will sell Onana. Keeping him longer, they will see his value on transfer market drops more.