I travelled by car from Calais to Aulnay 17470 via the autoroutes following Rouen/Le Mans/Tours. Viamichelin calculates the tolls as around 57 euros one way but my total came to around 67 euros PLUS TVA/VAT at 20% - so over 80 euros in total.
The charging looks joined up and consistent for a car (a class 1 vehicle), though one section was 35 euros alone (including TVA): A28 TLPC to A28 ORBEC SENS 2
Is it just one of those things? I'm surprised it's so hard to get an accurate estimate online in 2018. I did look at the websites of the toll operators, not great in my opinion.
The return journey seems to have been charged at around 21 euros plus TVA. Perhaps all the charges aren't in yet? Perhaps getting a bit lost helped....