The Hammers signed Payet from French club Marseille in the summer of 2015 on a five-year contract.
According to The Daily Mail, West Ham had to pay £10.7 million as transfer fee to secure the services of the attacking midfielder.
The 28-year-old has been a massive success in the Premier League and has established himself as one of the best players in the English top flight.
The France international attacker has scored six goals and created 53 chances in 15 league appearances for the London outfit so far this campaign. Sullivan has claimed that if Payet were 21 now, he would be worth £30m.
“If Payet was 21 he would have been £30m but he is 28,” the West Ham co-chairman told The Daily Mail. “Some clubs won't sign 28-year-olds. They look for investments. We are the opposite. We just want him to deliver for our club. We aren't looking to sell him at a profit.”
West Ham are sixth in the Premier League table at the moment with 35 points from 22 matches. Slaven Bilic’s side are just four points behind Tottenham Hotspur, who are currently fourth and occupy the final Champions League spot.