On one hand, the law does defines "prix d'achat effectif" (effective purchase price), which is the cost on the invoice, minus any sort discount obtained outside, plus various taxes and costs. In spirit, it's the price at which the seller would break even all things considered, though obviously the reality might vary on a case by case basis.
Code de commerce, Article L442-5:
Le prix d'achat effectif est le prix unitaire net figurant sur la facture d'achat, minoré du montant de l'ensemble des autres avantages financiers consentis par le vendeur exprimé en pourcentage du prix unitaire net du produit et majoré des taxes sur le chiffre d'affaires, des taxes spécifiques afférentes à cette revente et du prix du transport.
It should be noted that it forbids not only the sale below that price, but also explicitly forbids the advertisement of such sale:
Le fait, pour tout commerçant, de revendre ou d'annoncer la revente d'un produit en l'état à un prix inférieur à son prix d'achat effectif
The motivation behind this is that it's an anti-dumping and consumer protection measure. Larger companies could afford to sell stuff at a loss and drive smaller companies out of business, creating a monopoly where they could then raise prices at will, at the consumer's expense. More generally, French and European law aims to forbid unfair competition, monopolies, and other cartels.
The law 2020-1525 "Accélération et simplification de l'action publique" adds to the effective purchase price definition, which is where the extra 10% comes in, minus some costs and taxes still.
Loi n° 2020-1525, Article 125:
le prix d'achat effectif défini au deuxième alinéa du I de l'article L. 442-5 du code de commerce est majoré d'un montant égal au produit d'un coefficient 0,1 par une valeur P, égale au prix d'achat effectif défini au même deuxième alinéa minoré du montant des droits de consommation mentionnés au I de l'article 403 du code général des impôts et du montant des cotisations prévues à l'article L. 245-7 du code de la sécurité sociale.
This is defined under title IV of the law, called "Diverses dispositions de simplification", but I can't really tell if this was the simplification of an existing rule (as the name of the title would suggest) or not.
Regarding your question, the general idea is that your minimum resale price is buying price +10%-ish, but the 1€ and 1.1€ figures would be modified by a variety of costs, discounts, and other taxes.
On the other hand, "prix coutant" doesn't have a legal definition, or none that I could find at least. It clears the requirement that it wouldn't reasonably be understood as selling at a loss (doing or advertising so would be illegal anyways).
However it maintains an ambiguity on whether it means "the lowest price we can legally sell at", or "the lowest price we can afford to sell at", which may or may not be the same price. So it can be argued to be technically correct advertisement either way.
