I took my little fiat across last November (see Jonny Kirk's Seven Days.... article) and the ferry from Portsmouth to Le havre was definitely the cheapest offer going.
But just do an internet price comparison between all the various crossings. They've all got on line booking services.
Don't get drawn into a 14 day trip across europe though, it costs a lot in replacement parts when you get back. :wink:
14 days, 2000 miles, one 900cc engine!!!