Ugh. I live in Borderstan but have to drive to work in western Fairfax County (it is virtually inaccessible by public transport). The Commuter Connections section at Metropolitan Washington Council of Governments has an online calculator for figuring your monthly cost of commuting if you drive to work. Mine was far more than I estimated. With a 50-mile rountrip, my commute costs me about $550 per month. The calculator is here.