Original artworks have greater value due to scarcity. It's pretty simple supply/demand - if everyone wants artwork from a certain artist, those pieces become more valuable. The same reason the value of artwork tends to spike after an artists death - no more artworks will be produced, so there is a definite cap on the amount of art.

Mayor Bloomberg's statement that he intends to target Banksy's artworks to destroy them will make them very valuable indeed, in a "last chance to see" way.