Whilst I haven't used the route you mention, I used the route Strasbourg-Bordeaux a few years ago. At first I found the price seemed very high, but when I realised it included a breakfast at the arrival station and a car wash, set against the cost of a night's accommodation, motorway tolls and the stress levels involved, it seemed quite reasonable.

You'll need to contact French Railways for the actual prices ... but I'm not sure that the website actually covers this as Autorail is mostly a seasonal service.

The office doesn't open until November - I was just trying to get a rough idea of price to see if it compares with the ferry to Spain