You'll have at least four expense items in La Baule: airfare, lodging, transportation, and food. Do the math for yourself on the cost of a La Baule vacation:
Airfare to La Baule (Nantes airport) with a connection will cost 250-340 EUR round trip.
Average cost of lodging in La Baule hotels of 3 and 4 star categories is 80-110 EUR, in 5 star hotels - 200-400 EUR.
Rent a car will cost from 50-80 EUR per day. But it is cheaper to rent a car for a week - you can get within 200-250 EUR.
Prices for catering. Dinner for two will cost 40-70 EUR.
Plus excursions (from 20-60 EUR) and souvenirs.