700!
Finance was a big part of my decision to be a bike commuter; the math made it obvious that paying a few hundred bucks up front for a bicycle would eventually out-cheap spending a few dollars every day for public transit. (I never seriously considered spending tens of thousands of dollars up front for a car PLUS several dollars a day for gas and insurance, because come on.)
But I was curious, so I kept track of how much the bike had cost, how much I spent on maintenance, and the money I saved every time I rode instead of taking transit.
Today I reached a milestone: 700 rides past the point where it all paid for itself. Bike commuting has made me at least $2100 richer than I would have been otherwise.