Sullivan is also proud of their signing of Kevin Nolan.
"We are financing everything out of our own money," Sullivan told the Daily Mail. "Had we not put money in this year we could not have bought a player. Because there is not sufficient money to pay the debt. I cannot give confidential details of contracts but for the period of time Andy will be with us it's as expensive a player as we have ever signed.
"But our manager thought he was the most important player we could sign this summer and that is why we pursued him. Unfortunately he got this hamstring injury. But that's just bad luck.
"Against Fulham he made an enormous impact. He was the most important player on the pitch. He lifted the whole team.
"Kevin Nolan was a very bold and pleasing signing. We signed the captain of Newcastle when he had just scored 12 goals in the Premier League and convinced him to come to the Championship. It was expensive but he's the most fantastic captain, the most wonderful influence in the dressing room. And he's scored two goals in three games.
"We wanted to make a statement to our supporters, that we weren't going to run the club like an administrator. That's why we've continued to bring in more top players like Diame, Diarra and Jarvis. Others, too."