Unless my maths is really out (enirely possible, I'm not the best at figures!), I don't think it'll be too much dearer than running the IS for me. Petrol will make the most difference, but I'm finding I'm only <sigh> getting 30 miles less out of a tank of fuel.
I found it difficult to get decent insurance quotes, but when I did, it came in about the same/bit cheaper than the IS.
Awww, go on, you know you want one!