It's not a question of cost at all. The Nigerien mines are no longer profitable, and their cost is well above the spot price. Orano is literally subsidizing them. There was a project to extend one of the three mines, but Orano was not very favorable. It was pushed through by the French and Niger states for more political reasons. To perpetuate an important part of its economy for Niger, and diversify its suppliers and keep strong commercial ties for France.
As for uranium itself, French imports are not entirely destined for French consumption, far from it. France is a major uranium reprocessing country, so a significant proportion of its crude imports will be processed and re-exported.
Dans le cas de l'industrie lourde et la métallurgie cité dans l'article, c'est plutôt un enjeu de coût et de stabilité d'approvisionnement du gaz.