Your3DSource has a very neat list of world's most expensive animation cels (short for celluloid). This one is called "The Band Concert" and comes in at no. 1 as the most expensive ever:
"The Band Concert" was released by Disney in 1935, and is noted as the first color Mickey Mouse cartoon. In it, Mickey is trying to lead a concert of the William Tell Overture, amid various distractions such as Donald Duck playing the wrong tune on his flute, a pesky bee, and a violent storm. Despite these disruptions, the band keeps playing! This original cel was sold in a private transaction for a reported $498,000
USD.