Onana has been awful last season or should I say since he left inter Milan for the Red Devils, he hasn't been him self and for this concluded season he didn't have what it takes to go for him that's if the are to sell him. With Monaco going for him, I think the asking price is not suppose to be, for a goalkeeper that played 50 games in all competition and kept only 11 clean sheet, he should be sold for either £15m and not £30m...if I'm to say Monaco should look elsewhere for a goalkeeper because Onana has lost that spark in him or better still, they should see how he perform next season. If I'm to ask, is Onana worth £30m?
When Onana was bought by Manchester United from Inter Milan for £47 million, his performance at Manchester United was, as you said, poor. In 50 games, he only kept 11 clean sheets and finished 15th in the Premier League. So, a £30 million sale price was considered high for a player of Onana's stature. Even AS Monaco, who were initially interested in Onana, are now reconsidering his high price. Monaco will buy Onana when his price drops.
This again depends on Manchester United's management. If Onana is no longer needed, it's best to sell him at a low price. Maintaining his high price means no team will want to sign him. However, when it comes to underperformance, I think almost all Manchester United players performed poorly last season, not just Onana. Interestingly, whenever Manchester United players play for other clubs, they always impress. I mean, perhaps Monaco are interested in Onana because they hope, like any player, his form will improve when he leaves Manchester United.