As a fellow Scot I sypathise. i used to live in Aberdeen and it was a long, long 8 hour drive to Hull for the nearest ferry in the good?? old days. Now I live in Edinburgh so not a great deal better. I have noticed tho' a similarity between fares for Superfast, DFDS out of Newcastle and the old North Sea Ferries route from Hull. There would be no collusion between the Companies would there??
The further south you go the cheaper it becomes, down to market forces I suppose.
If you have 2 drivers then drive to the south and get the cheaper fares. If you have kids then I would suggest save up and pay it, the thought of 8-10 hours with the offspring in the back still gives me shudders. Maybe tho' I was slightly lucky. In those days I had a motorhome with a luton and we used to put the kids (5 - 8 year olds) to bed at the back of teatime and get a good few peaceful hours driving done until 11pm when we both crashed in as well. With a caravan you canot do that.
By the way have a look at Eurotunnel as well. They do some cheap single fares and it only takes 35 minutes to cross.