With the cost of a monthly MetroCard rising to $104 in January, we calculated that it would take 50 swipes a month to make the purchase more valuable than just paying per ride. But now there's an interactive chart! Capn Design mapped out the cost differences between the current $89 monthly MetroCard and the new $104 one, showing that with the new one you start saving at 50 rides as compared to 47 with the old one.
They also calculate that the average commuter takes the subway 42 times a month (21 work days, round trip), so unless you're doing a lot of weekend trips you might be better off with a pay-per-ride. Or just never going anywhere again, because once you get there there will probably be bedbugs.
UPDATE: It looks like the math was a little off on that first chart, but it's all fixed and the new one shows the $89 monthly MetroCard started being worth it at 46 rides, making the $104 card even less of a value.