Bill Watterson's original artwork from his surprise guest stint in June on Pearls Before Swine sold at auction Friday for a combined $74,040, with the proceeds going to the Michael J. Fox Foundation for Parkinson’s Research.

According to The Associated Press, Heritage Auctions sold the three comic strips to three collectors, who wished to remain anonymous. The Dallas auction house had expected the pieces to bring in more than $30,000 combined (the June 5 strip alone went for $35,840).

The collaboration with cartoonist Stephan Pastis actually came at the suggestion the Calvin and Hobbes creator, and marked his return to the comics page after a 19-year absence. Pastis teased readers that the week’s Pearls Before Swine storyline would “contain a mind-blowing surprise,” but didn’t reveal what it was. Nevertheless, some fans quickly uncovered clues that some of the strips were ghost-drawn by Watterson.

The artwork was displayed last month at Comic-Con International before it went up for auction. At Watterson's request, the strips were sold on behalf of Team Cul de Sac, the charity founded by Chris Sparks to honor Cul de Sac creator Richard Thompson, who has Parkinson’s disease.