Gas prices are currently $2.93 per gallon. Our 12 seat van gets around 9 miles per gallon. I am happy to stay in the 53 mpb category -- 53 miles per burrito. (thanks Fritz).
Other benefits to bicycle commuting that are typically bandied about include the health benefits. I'll just say I am the only one in my family who has thus far avoided the really bad cold that is going around. I have to believe it has something to do with a daily hour of fresh air.