the cost isn't high. According to my OBD-II device, my car would use 0.7L for an hour of idling, which is like 85 cents. My average goes up 0.1L/100km fairly quickly when I'm idling. I'm trying to get a tank under 3.9L/100km (60MPG) so I turn off the engine if I have to wait more than 30 seconds.