In a lot of banking and finance, they use "M" to abbreviate thousands and "MM" to abbreviate millions. It's based on the roman numeral "M", so "MM" is meant to represent "a thousand thousands". Of course if you know anything about Roman numerals, "MM" is in fact = 2,000.

I believe the appropriate abbreviation is "$425M", not "$425m". The AP Stylebook avoids the issue altogether and dictates that million, billion, etc. should all be spelled out and in all lowercase.