Fuel consumption depends on so many factors.
How ever establish your own miles per litre usage (eg how many miles you travel between refueling ) say 10 litres@£12 .00 and you cover say 60 miles,so for 6000 miles you need 100 x 10 lites refuelings, approx 100 X £12.00s.
Out there you will find many other ways of establishing a ballpark figure.
Cost per mile of a car isn't just the fuel, it often takes things into account like insurance band and average years servicing figures and such. However, if it is just fuel economy you're interested in have a look here: