Even the "minimum" number is not really the minimum. $15 / month is a fairly typical, average cost for birth control pills, not the absolute minimum. There are many places where one can get birth control for $10 / month or less. And their "minimum" assumes that the woman is on birth control every month for 30 years. Surely there are many women who go below this "minimum" by, for example, getting off birth control when they decide that they actually want to have a baby. And, hard as this may be to believe, there are actually some women out there who do not regularly have sex when they are not married.
Realistically, I expect the AVERAGE is more like $15 x 12 months per year x maybe 20 years (on and off) = $3,600.