When we were looking the costs varied for our 4x4 between £400 and twice that, single. They'd go up, down, sit still for a while, no pattern it seemed. We did end up paying a cheap rate, but we were fairly flexible and jumped when a cheap price came up.
It was also better to book direct rather than through Direct Ferries since £10 cancellation insurance was then available.
Bad weather on the Med meant it turned up in Barcelona about 8 hours late, so also arrived late in Tangier at about 11pm just as the insurance booth, money changers etc were all shutting up shop, and driving to the planned 1st stop would be in the dark.
I also thought there was only one company, one boat? So I didn't know there was a "fast ferry" choice?
Finding the correct unsignposted dock was interesting too