I use Shell V-Power Racing (100 Octane)
It's currently sitting at $1.55 over here (£0.72)
Standard Unleaded (95RON) is at $1.39 (£0.65)
So for a full 50 Litre tank I'm spending and extra $8 which equates to £3.73.
I didn't but this car because I was worried about running costs and I'm quite happy with the performance so if I don't get any extra MPG or grunt out of it I don't mind. (wouldn't object though

)
I am, however, more than happy to spend the extra to get a cleaner fuel that will keep my engine running better for longer.
Admittedly our fuel prices appear to be somewhat lower than what you guys in the UK have to put up with...
-H