In AusE, we use 'm' for million, 'bn' for billion: $25m, $25bn. Also 25k is used by some newspapers for 25,000.
Is that different elsewhere?
It's true that M (Mega) also means a million in SI units, as opposed to m (milli), but those terms take an affix, eg. MB (Megabyte), mm (millimeter), etc.