Each App Marketplace (App Store, Google Play, Android Appstore, etc.) keeps about 30% of the sales price for every app sold, plus any currency exchange costs associated with international sales. The term Post-Store-Proceeds refers to 100% of the amount remaining.
Of the Post-Store-Proceeds, the author is paid 71.4% (which is 50% of the sales price if the app is sold domestically in the U.S. and slightly less than 50% if the app is sold internationally due to additional exchange rate costs), and TaleSpring's receives 28.6%.
If your book app sells domestically for $3.99, the calculation is $3.99 (sales price) – $1.19 (App Marketplace portion) = $2.80 (Post-Store-Proceeds) , and your royalty is $2.00 ($2.80 x .714 = $2.00), which is 50% of the sales price.
If your book app sells internationally for $3.99 (U.S.), the calculation is $3.99 (sales price) – $1.19 (App Marketplace portion) – $0.12* (foreign currency exchange cost) = $2.68 (Post-Store-Proceeds), and your royalty is $1.91 ($2.68 x .714 = $1.91), which is actually 48% of the sales price.
* Please note that calculating an exact royalty percentage or payment amount based on the sales price is impossible for international sales, due to the fact that foreign currency exchange rates are constantly changing. The point here is that the App Marketplaces do not absorb currency exchange costs, they pass them on, so TaleSpring splits those costs proportionally with the TaleSpring account holder.
Since it is easier for most people to remember "50% of the sales price" than "71.4% Post-Apple", we often refer to the 50% number to simplify the royalty explanation.