In "Irrational Treasure," Quentin Trembley gives Dipper a negative 12 dollar bill as something to remember him by. Ford Pines also seems to have some knowledge about the bill, due to its appearance in Journal 2.
The bill's design and coloration resemble a typical USA dollar. An image of Quentin Trembley is featured in the center. The words, "NEGATIVE TWELVE DOLLARS," appear on the bottom and the words, "THE UNITED STATES OF AMERICA" are written at the top. On the top right and bottom left corners, it has the serial code. In the top left corner, it says "THIS NOTE IS NOT LEGAL TENDER FOR ALL DEBTS, PUBLIC OR PRIVATE." On the left, it has a Z and on the right it has an image of Bill Cipher.
The other side of the bill has the Eye of Providence on the right, above which "Semper Vigilantem" ("Always Watching") is written, and under which is written "Curia Infan-(obscured)." ("Curia" translates to "court", and various latin words starting with "Infan-" exist- "infandus" translates to "monstrous" or "unspeakable" and "infantia" translates to "inability to speak"- perhaps it's supposed to mean "Silent Court"?)
"It's less than worthless, my boy."