If you book a Norfolkline crossing between Dover and Dunkerque before 19 June 2009 through ferrycheap.com there are crossings available until 17 December 2009 from £19 each way in low season or £29 each way high in season.
Although the crossing is slightly longer to Dunkerque, the modern fleet lands you in a well-connected (if somewhat industrial) port which is well-connected for eastern and central France by autoroute. Even Paris is not significantly further than via Dover.
For more info on this offer see www.ferrycheap.com
For more info on Norfolkline see www.norfolkine.com
Best prices for fuel as at 3 June 2009 =unleaded €1.125 (£1.00); diesel €0.94 ($0.85) per litre.