Apple Upgrade Broken Down

Pick the device, pick your trade in, and then scroll and make the choices you would make. Watch the costs accumulate on the floating bar graph and notice which graph has the highest spend at any given moment. Pay attention to how they jump around and click on the months with big charges to see why.

In a hurry, or reading this on someone else's behalf? The short version has the rules that decide it, the catches that cost real money, and a JSON endpoint you can query directly.