This is more of a 'heads up' than a suggestion, but earlier today I was using Google maps to plot a route in the UK and as I scrolled down the page they have added an 'Estimated fuel cost' link that you can click on. It has 3 options for vehicle type, 'Compact', 'Standard' and 'High consumption'. You can then select from 'Petrol or Diesel' and alter the average cost of fuel, it then estimates a rough fuel cost for the planned journey.
It doesn't show up when creating a route in the US as yet, so I thought I would bring it to your attention in case it is of any interest or use as consideration for the Mapping programs. We repeatedly get asked "How much" in the forums.
[It may have already been suggested or on the agenda, as I didn't search]
Found a link to what I am going on about.