Sooz and I talked about the Big Drive she just completed.
It was clearly more expensive than flying, which would have cost her and Grant maybe £250 round-trip. In total, the bill for driving was at least £1000 for the ferry boats, parking at Stavsnäs, gas, etc. It also took 6 days, rather than the 6 hours of a round-trip flight. The drive itself was also unedifying; mostly freeways, and nothing really scenic.
On the other hand, Sooz and Grant got to visit Marcia's family (twice) and experience the vastness of the markets in Kiel. And we transported Ollie relatively stress-free, plus we carried a hell of a lot of stuff in the car.
Overall, though, we won't drive next year. Even if we bring Ollie, the total airfare for four of us will be cheaper than driving, with a lot less time spent in transit. And since our house is pretty much finished, we're in the happy position of needing to bring only food out with us, rather than our usual loads of building stuff and furnishings.