A friend of mine has broken this down precisely in this manner, and come up with a 'cannibalisation estimate' [1] of 50%, using product-wise sales data from the Apple-Samsung patent tiff [2].

[1] http://www.tech-thoughts.net/2012/10/ipad-mini-cannibalizati...

[2] http://allthingsd.com/20120809/apple-vs-samsung-trial-forces...