Currency

Currency in the 31st century, at least originating from the United States of America, seems to still be dollars. Most notes look just like they are today, and each one of them has the face of an important individual on it. The $1 note has George Washington, the $5 note has Abraham Lincoln,  the $10 note has Benjamin Franklin,  the $20 note has a Blob, possibly H. G. Blob, saying it is "legal tender",  the $30 note has Braino,  the $300 note has Richard Nixon,  the $500 note has Al Gore,  and the $1000 note has, once again, Richard Nixon. The coins also seem to be similar to what they look like today.

Notes and Coins

 * 25¢: Coolio
 * $1: George Washington's head
 * $5: Abraham Lincoln's head
 * $10: Benjamin Franklin
 * $20: H. G. Blob (possibly)
 * $30: Braino
 * $500: Al Gore
 * $1000: Richard Nixon's head
 * Special $300: Richard Nixon's head

Production
During the writing for "A Fishful of Dollars", the writers considered other forms of currency than dollars, including "quadloos", "bux" and "joules". "Bux" comes from the common 21st century nickname for dollars, "bucks". "Joule" is the standard measurement of in the 21st century. They finally settled on just keeping dollars.