I agree with the above. We don't use more than a million in this instance, i.e. we might say "Thanks a million," but we wouldn't say "Thanks a billion." Just a colloquial thing. From a mathematical standpoint, "zero point one billion," or "zero point two billion," would rarely be said outside of perhaps writing that made comparisons with other numbers that were in the billions (i.e. 1+ billion). For example, "Company X posted a 2.2 billion dollar profit in the first quarter, and a 0.5 billion dollar loss in the second quarter." This lets the reader quickly determine the total profit without having to convert between millions and billions: 1.7 billion. Most other cases, i.e. cases not involving comparisons, would probably state the number in hundreds of millions.

"Thanks a million" is all you need. We don't have a banking system set up for thank-yous. ;)

最普通的说法就是 thank you 还是 thanks。 照我的看法，前面的 形容词越多， 说话者的诚意越少。 虽然zero point one billion" = "one hundred million," 但我从未看过这样的写法。

This is funny, I think ( thank you ) is quite enough.or ( thank you so much )

There is lots of ways to thank indeed. However if you want to involve numbers, you would typically say : 'Thanks a (number)!' like: 'Thanks a million' And yes 0.1 Billion= 100 000 000...phew, too many zeros.