depends what car? a golf 2.0 TDI 100bhp you are looking at £4.80 (approx 8p p/m) but the misubishi shogun we run 2.5td guzzles about 20p p/m so that would cost ya a whopping £12
well factor in fuel, oil, depreciation, tyres, servicing. I'd say about £6000. If it's a diesel Ferrari.