Belly
From One Piece Encyclopedia
Belly (ベリー Berī; represented by the belly sign: 
) are the unit of monetary currency in One Piece, almost exclusively depicted in paper form, although coins do exist. Although closely resembling United States dollar bills, the large values on the notes harken to the Japanese yen. Assuming that one Belly is equivalent to one yen, that would mean that one hundred Belly are worth one hundred per US dollar. This would mean that Luffy's three hundred million Belly bounty is the equivalent of a three million dollar bounty.
It should be noted that because of the phonology, multiple english spellings exist; the most common among fans are is "Beli", although "Beri" is also used, "Berri" and "Belli" are used, but are somewhat uncommon. "Berry" for singular and "Berries" for plural is what is used by the 4kids dub and the VIZ manga. The author has spelled it both as "Berry" and "Belly" on the actual bills.
