Is it permissible for a wife to travel alone, without a mahram, to join her husband residing in France for studies, given that the husband cannot bring her to save time and money?

A woman is not permitted to travel without a mahram, whether by plane or any other means. Your wife can travel accompanied by one of her mahrams (a brother or other) for a fee, or you can accompany her yourself. Even if you lose the ticket cost, that is better than her traveling without a mahram.