Definitely get a Sanef tag, saves loads of hassle and you can then use the 30km/h lanes so you don’t have to stop. You also don’t need to worry about having change or finding your card etc.
Eurotunnel have the usual discount on getting a tag https://www.saneftolling.co.uk/Eurotunnel-Le-Shuttle-Offer-Landing-Page
Looking at our account the toll totalled £180 for our trip last summer.
We did Sheffield to Riems in one day then after about two weeks driving around set off back from Avignon. We stopped at about an hr from Calais on the way back at a F1 hotel which are fine and all a long the road. In hind site we could easily have carried on to Calais and made it back to Sheffield. The worst bit of the whole journey is the Folkstone to home. Driving in France is so easy, helped by Adaptive Cruise Control!!!
It does really help if the kids are good in the car and kept entertained with DVD’s etc. If they are quiet then the hours can fly by.