Interesting.  I had no idea, and I've BEEN working with financial data. 

Pretty much everything I've seen has used SI prefixes here in the US.  On the (rare) occasions where I've seen 'M' used to mean thousands I've briefly wondered whether it was a typo or a deliberate misstatement, but since the truth was clear enough for other reasons in those particular documents, never pursued it.

Anyway, I'm sticking with SI prefixes in everything I write.  It's the nearly-universal standard now and the sooner everybody stops using everything else the less confusion we'll have.  But now that I know some people have learned an obsolete system that will cause them to misinterpret SI prefixes, I'll at least make a footnote that says so.