He's only as valuable as someone will pay.
Character is normally as strong a criteria as ability, so I'd say his petulance will be a risk for a buyer. Not necessarily reducing his value, but certainly reducing the number of potential buyers prepared to take the risk.
IMO, the club see the sale of Pereria as the only way to offset the £23 million we lost in season 2019/20, that's why they've set a value of £25 to£30 million.
Financially, the club has made a profit if we get over £9 million.