West Ham co-chairman David Gold feels they allowed Dimitri Payet to leave on the cheap.
Marseille parted with £25 million to bring Payet back to the club, with Slaven Bilic initially insisting the player would not be sold, and Gold feels £50m is a fairer price.
“He's worth between £40-50m," Gold told BT Sport. "In the open market he'd have fetched at least £35m."
Gold is positive about his squad with the transfer window now closed.
"It is stronger in the sense that the morale is great now," he said. "There was a period of time, a couple of months ago, where morale was reducing.
"We had to make that decision, we didn't want him to leave and was desperate to keep him. The manager wanted to build the team around Dimitri but, sadly, sometimes it because impossible."
He added: "He's let the fans down, he's let the football club down, he's let the football club down and I'm hugely disappointed."