He doesn't decide the price tag but he went on with such transfers knowing they will be overpriced and will eat the majority of his budget, so when these players flop it's ultimately his fault for persisting with them.
Ten Hag options suck. He signed two over the hill midfielders as short term stop gap and now both look done. He signed Antony who has been awful, just because he managed him before. He signed Mount, whom he himself seems to have no clue how to use him. Malacia is a waste of space and now Onana doesn't even strike me with confidence anymore that he can still be considered a good business, looks an awful shot stopper. Up till now only Martinez has been a successful business.
He just added more duds to the club that we'll have to get rid of in few years time.