I don't know the price of diesel in the UK, but if this test were done in the US. The Prius would win in overall cost of trip if both mileage figures were about the same. Let me explain my (elementary) rationale.
87 octane petrol is less expensive than Diesel (at least where I live) with prices running $3.54 and $4.29 respectively. On a 500 mile trip that would be roughly about 10 gallons. The BMW would use about $43 (500/50*4.29) in diesel and the Prius would use $35 (500/50*3.54).
Personally, I would rather pay the extra money and drive the BMW but if economics were into play the Prius wins. It's cheaper to buy and diesel is now priced more than premium unleaded these days (It used to the cheapest). This simple fact may steal the_benefits_of_diesel thunder.
The originial article is here
http://driving.timesonline.co.uk/tol...cle3552994.ece