Those letters are actually the metric prefixes, also known as Système International prefixes, so they have the same meaning in every languages as unit prefixes, as in km for kilometers.
As in English, they can be used as shortcuts for the powers of ten, although this use is not as common as it is in English. In my experience, they are especially used when speaking of money amounts, and especially of remuneration, and M is less common than k
Comme professeur débutant, je gagne 2k euro par mois.
I don’t know about others abbreviations of this type.
Such abbreviations are hardly ever used in French.
k is the SI prefix for thousands, and it can be used as part of a unit (km, kg, kW, …). The prefix
M stands for millions — note that it’s a capital M, a lowercase m is milli-, i.e. thousandths. The prefixes are not normally used separately from the unit: one might write “30 mégatonnes” or “30 Mt” but rarely if ever “30 M tonnes”, where it would be common to write “30 m tonnes” in English.
Currencies are a bit different. In formal writing, they are fully spelled out, e.g. “150 millions de dollars” for “$30m”. In business contexts, it’s common to use the prefixes
M (sometimes in lowercase); I believe this is due to recent influence of English. For example (informal speach or writing):
Ce contrat nous rapporte 50k€. (prononcé « cinquante ka euros » or « cinquante mille euros »)
On a parlé salaire, ils m’ont proposé 30k. (prononcé « trente ka » or « trente mille »)
Le chiffre d’affaire a augmenté de 5M l’année dernière. (prononcé « cinq millions »)
I’ve never heard M pronounced, and I’ve never seen G used for billions in such contexts (it seems to be used occasionally in Canada). This may be because I don’t normally manipulate such large amounts, but then few people do. Even k wouldn’t be understood by everyone, it has a megacorp/finance feel to it.