You don't need to be Apple to work out that its a large number - the number of devices and cables bearing the MFI logo sold every year across the world is pretty nuts.
How much do you think Apple makes on certified devices - especially with all of the perfectly capable knocks off compared to the segments that they do break out - iPhones, iPads, services, wearables, etc?
Good question. The local Aldi here was selling charging cables the other day: micro-USB and USB-C cables were €2.99, the Lightning version €8.99; Same brand, length, etc.
While sure there's some extra margin on those cables it seems the cost of MFi certification seems to be more than just a penny per cable.
