I run an M reg diesel Astra and my Zed. Total insurance bill pa is £600 for both. I get around 19.5 mpg on average in the Zed and 45 - 50 mpg in the astra. Only MOT costs as I do the servicing with my mate.
I think it makes a big difference in the overall running costs in a year, well it leaves extra money for mods anyway!