If you have watched an animated tv show or movie that has came out before the mid 2000s, most likely it was traditionally animated using cels! The prices of cels depend on the rarity and popularity. For Example: Any animation cel from Spongebob Squarepants would cost you a pretty penny since the first season is only season that is tradionally animated.
Somewhere out there, there is a person who owns some of the cels from the sonic cd intro and i'm very envious. :'))