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
IT would be written as four hundred million.
$525 million USD
You would write 3.1 million dollars in numerals like this: 3,100,000