My scangauge will pay itself off within 9 to 10 months (assuming gas stays over $4/gall)
Here was the article on idling times:
Is it more efficient to leave your car idling? - By Brendan I. Koerner - Slate Magazine
...but it was in terms of a 6cyl engine so that might make a difference -- how much, I don't know. I just go by the 10sec rule (and make sure its not illegal to shut off your engine while stopped at a light). So far, I've shut off at a red light once because I've been getting better with timing my arrivals at stop lights.