How much is One Piece's currency worth?
In the original, it's called Beli in English. However, if the Japanese word is translated correctly, it's Beri - it's a feature of the Japanese language (look at SingerOfTheFalls answer). However, it differs in different languages: for example in Germany, it's Berry.

To compare the prices:


One cabbage loaf: 150 Beli
A Bon Chari: 500 Beli for renting, 10,000 Beli for buying
A Doskoi Panda brand shirt: 10,000 Beli
Nami's Jacket: 28,800 Beli




Regarding Luffy's bounty would be an equivalent to 30 million Yen,


   30 million Beli on Luffy's head is a little bit old; his current bounty is 400 million Beli.


Quote:


   Fourth Bounty: Defeating yet another Shichibukai (Gekko Moriah), attacking a World Noble, causing an unprecedented breakout from Impel Down, participating in the Whitebeard War, and by intruding Marine Headquarters a second time and ringing the Ox Bell 16 times (which was taken as a declaration of war), Luffy's bounty has gone up 100,000,000 more to make a total of  400,000,000 making it the third highest active bounty known. The public revelation of his relation to Monkey D. Dragon may also have influenced it.