Slopes, I used to do this regularly before speedferries went tits up - it's the only option from bolougne. It's certainly cheaper, but how much I can't say as it's been a while since I did that route, and all my recent experience of the A1 route is in a van. Prob worth 10 Euros or so though. Keep in mind that it'll certainly be more than 12 mins slower if you're in a vehicle and driver inclined to "make time" on the autoroutes as the A16 route gets a bit suburban and traffic light heavy on the approach to the BP.
Overall I think it may well be a good option if you're in a large slow van (so more savings, less time loss), but not worth it in a car as it'll prob cost you at least 1/2 hour in journey time.
