I accumulated 520 miles, on 30 riding days, in April. I'd guess 40% of my riding was work-commuting, 10% was running errands, 50% was "recreational."
How much did it cost to go those 520 miles? Gas - $0. Parking - $0. Insurance - $0. Registration - $0. I spend about $60/year on tires - so let's figure $5 in tire cost. Maybe $4 for a new tube and a couple patches. So... $9?
If I'd been driving a Prius instead of riding my bike, gas (just gas) would have cost about $28.00 (figuring $2.80/gal). Honda Civic - $48.50. GMC Yukon, or Ford F150 pickup - $104. (Ouch!)
I also rode 424 miles on my motorsickle. (About 380 of that was this past Saturday, on a little tour of eastern Oregon mountain roads.) I figure it cost about $35 in gas for my pleasurable m/c pastime.
(It's now been over 18 months since I used any mode of transportation, other than bike, to get to and from work. I've ridden my bicycle on 228 consecutive days - every day since September 14.)