It is calculated using the same blend of prices by which IDC determines the average software unit price, including: retail, volumelicense, OEM, free, open-source, etc.
This is why large companies strongly prefer to buy hundreds or thousands of apps at a time via a volume software license that is paid for via purchase order, not credit card.
With HP's primary play in webOS being outside the smartphone market, it creates more room to license webOS in that high-volume category --a place where the OS really needs to attract developers.