Premium gas here is about 10 cents per litre more than regualr gas. Suppose you drive 20,000 kilometers per year, and average 9.5L/100km's in the C230 (my city driving mileage)
Over the course of the year, you'll burn 1900 litres of fuel. At a $0.10 premium for premium (get it...) you spend an additional $190.00 per year.
190 bucks? In the overall scheme of maintaining and owning a Mercedes, $190 per year is chump change. I wouldn't bother.
1998 C230 "Black Betty"