You can very easily find (with a bit of Googling) what your particular mode of transport's fuel efficiency is in miles per gallon. If you want to, you can find data that shows urban, extra urban and combined metrics. You can then quickly work out how much each mile will cost, since you know what the price at the pump is and where you're driving from and to.
If you have weighed up postage and packaging on some Internet shopping site and decided to go ahead and drive to get said product instead, it could well be worth calculating how much you will spend in petrol, especially if the drive is significant. It's a very simple thing to do nowadays with the resources available, such as Petrol Prices in the UK (which givens you a range for your postcode) and Google Maps.
You also might want to do a quick calc on how much you spend a year on driving to work. I was surprised.
One thing I would love to see in the future is some sort of Google Maps mash-up that would give you the fuel price of your journey as well as the distance, based on your login profile. It could get very clever in working out your cost based on the type of road and the average traffic at the time of day.