I've done the journey four times by car from Calais or Boulougne.
Dover - Boulougne on Norfolk Line is much cheaper than Calais using P&O or Sea France
If you drive near legal speed limits you can do this with two full days of driving and one night in a hotel
We stop at Clermont Ferrand which is sort of the middle of France and where the North South and East West motorways cross. My son has done it a number of times and he does it without an overnight stop and with short breaks he generally takes 24 hours to Aylesbury.
I always use the http://www.viamichelin.com
web site to plan the journey.
If you enter your mpg for your car it will give you an accurate and up to date costing including the tolls
You can then balance, fastest, shortest, avoiding tolls etc etc routes quickly and easily. You can then print out a route guide for the journey.
The motorways are generally easy driving and interesting - much better condition than our roads, the only tricky part is getting round Paris, so if you have or can borrow a SatNav this really makes this part much easier, we usually try and do this before 7am or on a weekend morning. Many of the newer SatNavs have the speed cameras listed and sound an alarm when one is coming up. The ViaMichelin routes mark the position of these speed traps.
I wouldnt dream of taking one of the longer ferry crossings its just too expensive and time consuming and even when you get to Spain its still a fair drive down from the North.
I have a spreadsheet somewhere with all my costs, but to be hionest ViaMichelin calculates it all for you using upto date prices