Fast Times at Ridgemont High I

VH Scene 1: In the beginning, two kids approach Damone about Van Halen tickets. He says they are 20 bucks apiece, and the kids say that the face value is 12.50 each. After Damone says take it or leave it and the kids purchase, Damone looks at Rat and says ” I just made 16 bucks!”  
 This is a known movie error….. If the kids paid $40 for something that was $25– shouldn’t he have only made $15?