Last year's Britain's Got Talent winner told The Sun that Cowell was more interested in his music acts, such as X Factor winners Alexandra Burke and Leona Lewis.
Sampson said: "Things aren't going so well with me and Simon. I suppose he's realised he can make more money out of other people.
"But I thought we were tight. I had one of the best-selling DVDs at the start of the year. We sold loads of copies."
He added: "But I've just found out that Simon's company takes 90% of the revenue. I get 10%. But I have to pay the production costs out of that.
"So I ended up owing them money. I'm not quite sure how it all works but that can't be fair, can it?"
Sampson was also reportedly upset that Cowell purchased a £6,000 Chanel handbag for Burke's 21st birthday, while he gave him £150 of shop vouchers for his own 16th in June.
He said: "All he got me was vouchers for Harrods. What am I supposed to buy in there?"