Speed Ferries are offering 'open' one way tickets for £30 each way between Dover and Boulugne for a car and upto 5 passengers. The great thing about these tickets is that you can use them at ANY time upto the end of 2009. So even if you're not sure of your exact dates, you can buy the tickets now and confirm them later.
Having checked their website, it appears their most expensive return in peak season is £158, therefore using this deal can save upto £98 for a return journey.
I was planning a last minute return to France this weekend anyway and using this deal saved myself 25% on their cheapest ticket. Speed Ferries are also the quickest way to cross the channel with a journey time of only 50 minutes. Boulougne is also the best route for trips to Paris and Euro Disney.
The website does not give a closing date for the offer, but does say that it is a limited offer, so get in soon!