In the harbor/Port:
Everywhere you see signs with '1 kg. Moules and Frites' for just € 8,00 per person. That sounds cheap (well.. in Cap d'Agde it is..) but first take a look at the chart before you sit down.. in some cases we payed more than € 6,50 for a Coca Cola or a beer.
Unique Suggestions: The places with life music are the most expensive to drink something, if you just want something to drink and could care less about the view; there are 2 supermarkets (Hyper and Spar) in the center, just walk on the 'Flanerie du Mole' (from the harbor to the beach) still expensive but 4 cans of 0,5 cl Heineken will cost you about € 5,50 instead of 4 glasses of 0.33 cl for € 25,00 !!
If you do wear clothes to the nightspots of the naturist quarter in cap d'agde, I suggest you don't wear your best cashmere cardy!! There are pole dancers on the bar bench tops and they do squirt the crowd with postmix machine nozzles! Highly amusing, but lemon squash doesn't do great things for high cost fabrics!