Japanese Sign Language for "Money" is easy to remember.

It represents the old Japanese gold coins called "Koban 小判" (Not to be mixed up with "Kouban 交番" Neighborhood Police Station.)

For many hundreds of years, Japan was based on a Rice currency.

Farmers paid taxes with Rice.

Samurai were given lands and collected Rice.

Purchasing items was done with Rice (or Rice wine).

However, in the 1550's, when the Portuguese who came to Japan, they preferred Gold over Rice.

After 400 years, this led to Meiji Government sanctioned Koban and then eventually to Yen.

For more information:

http://en.wikipedia.org/wiki/Koban_(coin)