I've done some math and it seems that riding a bike for the past eight years has saved me $9,000 in TTC fares.
I'm looking into it because I've had trouble with my bike's center-pull cantilever brakes of late; the 700-38C tires appear to be too big for caliper brakes, so a decent pair of v-brakes is in order. Which means new brake levers. I sometimes feel that owning a bike is asking for a non-stop stream of expenses, but: 1. The brakes on the bike are already 4.5 years old and have seen 18,000 kilometers. 2. The brake levers came from a previous bike and probably have seen 28,000 kilometers. 3. Since I started keeping track in early 2011, I've spent $6,237 on two consecutive bikes, and saved $9,065 in fares.
My methodology on costs is simple: record all costs. My methodology on expenses is a bit different: when I use my bike instead of the TTC, I record the trips "saved" and multiply that by the then-current cost of a fare.