There are still some medium/small cities in france near the sea that have low-ish rent/cost of living. You just have to look way north of the mediteranean sea. I'm currently in Douarnenez, in britanny and pay 560€/Month for 80m².

Considering the weather we had this summer, i'd say you would be happy if you liked the heat. Maybe look for rentals on the north coast of britanny?