It's a shame the camping has become quite expensive now for a family of four. We are staying two nights in a FEWO mid October, costing £180.
Tunnel out is £115 (booked it quite early though) and returning on Irish Ferries which is £70.
Overnighting in a B&B Hotel, £74 on the way, £78 on the way back.
It's around 850 miles Calais-Europa-Calais, we usually avoid tolls and fill up in Luxembourg. In summer it was 1.6 euro per litre of diesel, France was more than the UK at the time.
There's a couple of tolls maybe worth paying depending on which way you go. The tunnel Maurice Lemaire is 6.30 euro, between Selestat and Nancy, and knocks 30 mins off the 'toll free' journey.
Via Strasbourg A4 to Metz is 14.60 euro and is another 30 mins quicker.
Via Karlsruhe is free and an easy drive but uses more fuel, in my case about 10 euros, so it's cheaper to pay the toll as it's not as quick as using the A4.