Readings & Meanings:
The wartime slogan "Fight Until All Hundred Million [of us] Die with Honor!" is now completely absurd.
If you had 100,000,000 yen, how would you use it?
人 on the left, otherwise known as Nin-ben.
On the right is another JLPT kanji, 意. We've seen it in 意味, 意志, etc. It means thoughts. Look at the components of 意 for a minute. 音 (sound) is above 心 (heart).
Let's digress further. 音 is a twist on 言. 言 is clear, fluid speech, with the 口 character at the bottom free to pronounce words clearly. 音 is the same kanji, but jumbled up, because the 口 element has something in it (a horizontal line) preventing clear speech. Therefore, 音 is unclear speech. Therefore, 音 is sound.
To get to 意, the character for thought, look at the bottom of the character: 心. So 音 shows blocked words, but 心 indicates that the words are so blocked, they aren't even spoken aloud. 意 is unspoken words, or thoughts.
So why does 億 mean 100,000,000?
No reason. The Chinese just picked that kanji once they had a need for that big of a number.
Okay, it wasn't all a wild goose chase. Long, long, long ago, 億 meant "person who thinks great thoughts". It's never used that way now.
億 is a good way to express "a lot," "many," or any vague, huge amount. 億万長者 is a word for billionaire, trillionaire, or any filthy-rich person. You find this word mainly in mukashi-banashi (old stories) when one character is the rich man of the village.