Google has added a fuel cost calculator to driving directions on Google Maps in Europe. At the moment the estimated fuel costs seem to only appear in driving directions in Europe and not in the USA, Canada nor Australia.
The fuel cost estimation appears at the bottom of the driving directions in the map sidebar. For example, if you are driving from Manchester to Edinburgh and scroll down to the bottom of the driving directions you will find a link to the fuel cost estimates.
If you click on the estimation a calculator opens allowing you to adjust the price per liter, the type of vehicle you drive or switch between petrol and diesel.