Etymology: Like so many kanji characters, this one has evolved from its original form and meaning.
It's original meaning had a character meaning round kettle, contained within an enclosure radical 口.
This evolved to its current form, expressing circle, which also means money.
HINT: Although not accurate, think of this as a pictograph of a bank teller's window.
Mnemonic: ROUND COINS FLOWING FROM BANK TELLER'S WINDOW.
円高 (endaka) strong yen
百円 (hyakuen) 100 yen
(Based upon Henshall, 1998)