These are some of the places I considered and the prices I found when looking for a safe marina in which to winter my boat in the Western Mediterranean. Prices are for a 12m boat, in the water, unless otherwise stated.
Gruissan, France
around €193/month
Barcelona, Spain
Port de Olympico
around €160/month
Mallorca, Balearic Islands, Spain
around €500/month
Menorca, Balearic Islands, Spain
Ribera del Puerto
around €160/month
Napoli, Italy
around €360/month
Other options
Costa Almeria/Costa del Sol, Spain
Formentera, Balearic Islands, Spain
Ibiza, Balearic Islands, Spain
Alicante, Spain (full when enquired)
Gibraltar
Nice, France
Sicily, Italy
Valletta, Malta
Tunisia