Gas costs me about €0.07 a kilometer. Add maintenance, and you're at maybe €0.10 a kilometer.
If parking fees at both airports are the same, you can calculate when it's cheaper to take the higher fare.
Of course you have to do this at both ends of your trip.
If you're flying into another airport on the lower fare which is further from your destination take that distance into account as well.
If you're on a business trip, take your hourly rate into account as well (for me that would be about €100 an hour) for both the transfer and difference in flying times.
In the end, the higher airfare might well save you money.
For example flying AMS
is often effectively cheaper than flying AMS
on EZY because the transit cost from LGW
to the city is so much more expensive (in time the price) than the cost from LHR
. This can amount to 15 pounds for a return trip, while flying EZY is often only about €10 cheaper than BA