Hey folks this is when a practice or reconnaissance trek is a great idea: if you know how much fuel your vehicle uses for a (for instance) 400km trek, you can budget for a 4000 or 40,000km trek over similar terrain, based on your figures
Keep in mind remote areas charge a LOT more for fuel - others may know recent prices...?
Food - you need to eat at home, so that is not a 'cost' as such, but you will need to take the bucks with you, of course.
And if you want to be really thorough, add the cost (as you mentioned) of a service and tyre wear, too
Hope this helps