What is the salary range for a Post Doc in France?
I did two post-doc in France, both paid by public institutions.


In 2013, CNRS paid 2.022€ net per month,
In 2014, INRIA paid 2.120€ net per month.


This included social security, but I had to pay some taxes at the end of the year (so beware and save!).

I also had access to various benefits, both from my employer, e.g.,


re-allocation funds,
contributing to the cost of my commuting (this is a legal obligation, and they will happily comply),
financial help to pay a nurse for my kid


and from the state.

Those offers were "forfait" and not negotiable.
Most of the public employers will be happy to provide you with additional information about the various benefits you can access to, and to discuss the cost of living, but they have absolutely no room to discuss your salary.

If you look at the INRIA's webpage, they explicitly give the salary for a post-doc with them: there won't be no exception, everyone gets the same salary, no matter the town, subject, experience, qualification, PI, etc.

I don't know precisely the situation at L'école des Mines, but I would be surprised if the situation was much different. This page reads


  Pour ses actions de recherche et afin de conforter les liens très
  forts qu’elle entretient avec les entreprises françaises et
  étrangères, l’École recrute également de jeunes scientifiques,
  titulaires d’un doctorat ou d’un diplôme équivalent, 
  
  
  sur contrat à durée déterminée, éventuellement renouvelable
  leur assure une rémunération brute mensuelle allant de 2 340 € à 2 944 €.(chiffre 2007)
  leur permet d’acquérir au sein de ses laboratoires de l’École une expérience complémentaire de très haut niveau dans les métiers de la
  recherche
  


So, in 2007, the salary range for a post doc was between 2.340€ and 2.944€ but I would assume that on what ends of that spectrum you land depends on your age, experience, social status, and not on negotiation. I could be wrong: the better way to get a definitive answer is to ask them!