Cahill has been touted as a possible successor to Tony Roche, who parted ways with Federer in the lead-up to the French Open.
“I don’t think he (Federer) would have enough money to pay me,” Cahill told television station ESPN.
Cahill, who has been assisting Hewitt in his preparation for the Grand Slam event, has also ruled out returning as his full-time coach.
Cahill helped take Hewitt to No.1 six years ago and has been working with him on a casual basis in recent weeks.
But he said his Davis Cup and commentary duties prevented him from making a full-time commitment.