The first question isn't which ferry is cheapest. It's where you actually want to end up once you're in France.
Start with the bit after the ferry
If you're driving towards Lille, Belgium or further into Europe, landing at Calais can make perfect sense. For a Normandy break, Caen may be a better starting point. Headed for Brittany? A crossing into Roscoff could spare you hours of motorway. A route that takes longer on the water isn't necessarily longer overall.
The bargain that isn't always a bargain
Imagine saving a little on the ticket, only to spend it on fuel and a hotel because the ship lands miles from your first stop. Look at the final driving distance, the time you'll be off the ship and whether you'll need a cabin. That's a much fairer comparison than the fare alone.
Make the first afternoon enjoyable
A crossing rarely ends the minute the ferry docks. There can be a wait to disembark, then a drive on unfamiliar roads. I would leave time for lunch and a comfortable first stop rather than booking something with a tight arrival deadline.
Routes to compare
Explore the destination
A few things you might be wondering
Which route is fastest?
Dover–Calais is the short Channel option in the current directory. Compare the whole door-to-door journey.
When should I book?
Compare once your dates and vehicle are known; popular dates and accommodation can have less availability.