Brag about what? Please take a moment to read my comment with understanding.

And the dollar sign goes on the side I put it: nine hundred thousand dollars. The fact that you want to write $900k is just an idea specific to the finance world (such as using parentheses for negative numbers).

Do you usually write m4, in7 or lb9?

Dollar sign goes before the number, cent sign goes after, other units go after: $7, 7¢, 7 lbs. You can’t pretend convention doesn’t exist just because it’s weird! That’s why it’s convention and not common sense.

imho most countries on earth use the format: [scalar] [si-prefix] [unit]

Currencies are formatted differently in different locales, even foreign ones.