'M' is the roman numeral for thousand, so 25m traditionally means 25000. mm is thousand thousand, aka million.
It seems odd that anyone would think $25m should be read as $25000. It's much more likely that I would read $25mm as 25 millimeters instead.
Capital M has traditionally stood for both thousand and million. In other words $20M and $20m is ambiguous, depending on what tradition you come from. $20MM and $20MM and $20k are not ambiguous.
$20M = $20,000
$6MM = $6,000,000
$30k = $30,000