Sports Illustrated did an excellent feature video on how the Phillie Phanatic came to be, going all the way back to Bill Giles hiring the great Jim Henson.
SI brought Bill Giles, Mike Schmidt, Larry Bowa, the men behind the green mask, and the Phanatic’s creator/mother (?) Bonnie Erickson.
The original Phanatic cost $3,900 without the copyright to the character. With the copyright it would be $5,200. Bill Giles decided to be cheap (somewhat shocking) and opted to let the Henson Company keep the copyright to the Phanatic.
A few years later, he paid $500,000 for the copyright.