We paid over £900 to Santander return on the Pont Aven last August, for 2 adults and 2 toddlers in one Coffin cabin. It was HELL!!!.
Our plan was to drive to just above Biarritz for a camping trip – we estimated about 2.5 hrs, but that ended up being 5.5hrs.
On the way back the underside of the Pont aven was slapping down on the waves meaning no sleep for anyone. It is an experience I wil never repeat with young children. Speaking of which, the only thing for young children to do is a crappy little room with broken crayons and a few colouring sheets. there's also a climbing frame that was permanently invaded by surly pre-teens thinking they were hard and cool, and intimidating the toddlers. Also, the scooby Doo movie on a constant loop was too scary for a 3 year old and 18 month old to watch.
Having done the journey from calais to SW France (landes region) on many occasions, if you have very young kids with you, the road trip is actually easier. The main part of the journey is on a dual carriagway N route without tolls, but with plenty of Aires.