I've been looking at the prices for the Portsmouth-Le Havre ferry (not that I'll need one

) and it pays to play around with the times/days.
If you use LD lines' own site,
http://www.ldlines.co.uk/ the price results come up in a nice chart showing variations for days/times there and back. Returning on the sunday seems to be the most expensive, however sticking with that, the price varies depending on when you leave; (prices are for 2 people, 1 car) (1 person, 1 car @£20-£30 cheaper)
Leaving on Mon 10th at 8:30am (fast ferry) - £198.
Leaving on Mon 10th at 23:00 (overnight) - £172.50
Leaving on Tues 11th at 8:30am (fast ferry) - £134.
Leaving on Tues 11th at 23:00 (overnight) - £124.
Which still leaves time to get to the site before it opens at 4pm on wednesday.(this is the cheapest option on this route that still covers the whole meeting)*.
If you travel out on the Monday, then coming back on the following Monday, rather than the Sunday, will save about £11 or £12. However if you travelled out on the Tuesday then staying till Monday can work out over £40 more!
No, doesn't make much sense to me either. You pays your money, you takes your choice.
*The longer your stay in France the more the crossing costs. LD lines are a french company, they don't want you in their country any longer than necessary!