Well, not to piss on anyones bonfire, but there is a p&o promotion running where you get a day return for £20. So, book two of those and you have a return for £40. You might be thinking that that isn't such a good offer, but here's the rest. On each journey you get 6 free bottles of wine (approx £20 value), and a free meal (£8 value). It sounds rather unbelievable but I just came over yesterday on this very trip. It was all a bit sketch since I sneaked onto an earlier ferry after being told the ferry left the port (LIES!), and then I had to pretend I was actually coming back (which meant I wouldn't have been getting off the ferry!).
what so you're going to claim that you stayed on the ferry for days/however long your trip is? whatever. how the fuck is that going to work.
I like speedferries - not quite as cheap as NL, but still bargain basement, and Boulogne is a fuck load easier and quicker to get out of than Dunkerque (ie <5mins to Autoroute). A16 route is cheaper on the peage as well.