I currently own a 1997 Ford Falcon station wagon. I estimate it will use about $1200 in petrol to travel 10,000 km versus $600 for a small car. It would be less with frequent highway travel. It is a manual so is more economical than the automatic version.
How about other running costs? Like tyres, oil, and other parts? How would it compare with an Hyundai i20, Toyota Prius and the Suzuki Swift? What about an old small car like Mazda 121 and Ford Festiva?
I enjoy having the station wagon because I can carry a lot of gear and sleep in it. If I decided to buy a smaller car and camp, how much would I save?
Edit: Update
Falcon, especially older Falcons, are pretty cheap to maintain and repair. I would guess about the same as a similar vintage small car, assuming an equally common car. Tyres are probably a bit cheaper for a small car, and they might use a little less oil.
I think you would find maintenance on a Prius substantially higher as the batteries in the earlier models are reaching their life expectancy.
i think the main saving is in fuel. At current prices, you estimates are probably fair. Remember the difference adds up every year. Based on the value of 19 year old Falcons and Mazdas, you would be in front after 12 months or so if you swapped, allowing for transfer costs.