Cost of Raising a Child
Estimated spend per child was sourced from the Consumer Expenditure Survey by the U.S. Department of Labor conducted in 2005-06. The survey provides an estimate for the annual cost of raising a child in a dual-parent family up to the age of 18 for three differnet income brackets. This data is summarised on Wikipedia.
Estimating Cost per Month of Childhood
Estimates of annual child expenditure were divided by 12 to estimate a monthly amount. This amount was then adjusted by U.S. Department of Labor Bureau Statics published CPI (All Urban Consumers) to estimate the costs incurred in specific months. A table of this data can be found here.
Estimating Shares Purchased
Adjusted closing share price for Apple was sourced from Yahoo Finance. Prices have been adjusted for stock splits and dividends.