Coming back to Calais I always try to come off at the stop before the tunnel and fill up with diesel. The service station underneath Cité Europe couldn't be handier, although it's a bit hard to find. The road takes you past the Eurotunnel admin HQ on the left - look to the right and you should be able to see the queues for the train, or no queues if all is going smoothly. The French like to keep you penned up in the terminal where you might possibly buy stuff until the last few minutes.
Go round the roundabout, and a 100 metres on the right there is a curious double exit. The first is for buses and delivery vans and closed off by a barrier to mere mortals. The second, which is a bit tricky to spot, takes you down to the basement level of the car park. Follow this towards the exit and the service station is on the right. You can pay at a kiosk in the week; on Sundays and out of hours the pumps all take credit cards.
There's a good reason for filling up here, whichever way you are going. It's a lot cheaper than the motorway service stations (but you knew that already). And it's a lot cheaper than UK diesel, because the French taxman doesn't clobber the diesel driver as much as our beloved Chancellor of the exchequer. A litre of French diesel is £1.05, compared to around the £1.35 mark in the UK.
Petrol? I would have to check - my last couple of cars have been diesel, partly because of the French pricing. But I reckon that Unleaded is much the same price as Sans Plomb