It is a slang term that refers to US paper currencies. It can be any unit of a currency but it is most often refers to George Washington where he is the face of the $1 bill. Therefore an item that costs six dead presidents would mean that it costs $6. During the Civil War, the Union Congress ordered the printing of half a million 5-cent notes due to a shortage of the metals used to produce traditional coinage. However, when the notes appeared to the public, the engraved portrait on the bill was of someone named Spencer M. Clark. Naturally, no one in the Union (or anywhere else) had any idea who this was. An investigation quickly revealed that Lincoln had appointed him as the first superintendent of the National Currency Bureau.