I would imagine it's down to mileage, and that they are starting to realise people who run 20+ year old cars like cossies tend to look after them, so less time on the road is less risk from other people's actions.
The astra costs me £130 a year and the cossie around £500, but I have unlimited mileage, and can do 1000 a week in a very busy week, although I love the oomph I actually prefer the astra 1.4 auto, as I'm fed up changing gears now!!
tabetha