I run at 17.2pence per mile (over 1 year). That is for a 4.3litre beast of a lexus, but running on lpg. Cost could be less if I didn't occasionally have to fill it with lpg at the local garage which is 10p per litre more than the ones near work.
I do almost no motorway miles, and only 12 miles or so of dual carriageway on my commute, but over 50 miles I rarely drop below 60 - a couple of junctions, 2 sets of traffic lights, and a toll bridge.
In terms of costs of diesel v petrol (or indeed lpg) that assume syou are buying new. My car was £66k new (still have the receipt) - I paid well under 10% of that, and it came ready converted. In my mind that makes for comfortable and relatively cheap motoring.