Beli
From One Piece Encyclopedia
Beli (ベリー, Berī; represented by the Beli 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. With a beli about the same as a Japanese yen it would make Luffy's 300,000,000 bounty about $3,025,500.06 and Chopper's 50 bounty around $0.50 when converted into U.S. dollars.
It should be noted that because of the phonology, multiple English spellings exist; the most common among fans is "Beli", although "Beri" is also used. "Berri" and "Belly" are used too but are somewhat uncommon. "Berry" for singular and "Berries" for plural is what is used in the English versions. The author has spelled it both as "Berry" and "Belly" on the actual bills.
Nami's eyes tend to take the shape of the symbol of Beli when she hears that a large amount of money is involved. Especially when explaining that what she is doing has nothing to do with the money.
In the SBS of the volume 53, Oda showed how are the coins and notes of One Piece:
