Mandaric took over in the Owls boardroom at the end of last year, paying £7million to the club's bank and footing tax bills of around £1.4million.
He told the Sheffield Star: "I had to settle a lot of bills, there were also managerial contracts, plus losses during the season that we had to deal with, and there was a couple of million to bring in players.
"I have spent at least £16million already, and by the end of the season I think the total will be between £16million and £20million.
"I don't call that investment a loss because I know it is getting the club in the right direction. We are a club with ambition."