You would write it: 525,000,000
525 = 525/1, although I see no reason why anyone would wish to write it like that.
525 = DXXV
525 = quinientos veinticinco
$525 million
I would write it as 2.06 million - exactly as in the question.
To write 20.5 million numerically, you would write it as 20,500,000.
2400 million
You would write it as.... 34,500,000
You would write it as 60,000,000
$525 million USD
In its simplest form 525/1000 = 21/40
IT would be written as four hundred million.