We don't use Roman numerals for thousands, why should we do so for millions? The right way is obviously using metric prefixes, so 1k$, 1M$ and 1G$.

And don't forget that 'cents' is short for 'centidollars'.

Is there a special reason the 'k' is lowercase?

Well, M for mega is capitalized since m is already used by milli. And all of the SI prefixes larger than mega are also capital, I guess to denote that they're large...

All of the SI prefixes k and below are lowercase.

Exactly, capital M

I've always seen k used to represent one thousand in dollar amounts, so I've never interpreted M as being a Roman numeral but simply as standing for "million."

Disclaimer: I've been in the US my whole life.

But, then that would be 150 * 2000 = 300,000.

Yes. If we're talking Roman numerals, MM would be 2000, just like XX is 20 -- not 100. This notation makes no sense to me either.

Exactly. 1 million in Roman numerals is M with a bar over the top of it.