Parking: 3.75 per day x ~20 work days per month: $75
Gas: 35 miles RT (via car) at about 20 MPG when stuck in traffic: 2 gallons per day or 40 gallons per month to the tune of $130+
Subtract the monthly bus pass fee that I pay (the company pays for $60 of it, I pay for the other $15):
I've been keeping track of bike maintenance too, which for me has averaged about $20 per month since January of 2007. Chain, brake pads, cables, occasional tools, bearings, chain lube, degreasers, tubes, tires, etc...
+$ 75
+$130
- $ 15
- $ 20
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$170 saved every month, give or take.