One of these gadgets can supply up to 100 Books already loaded depending on format, that said lets look at the cost implications of a standard book in South Africa compared to a laptop at say $200.

$200 equals roughly R1000, one Book = +- R100 to R200 if it were say a fiction book, non fiction books could go up anywhere around R500 (note that books more than R200 are usually kept within the library Reserved Section)

That said if one were to simply look at the Economies of scale if this "gadget" stored just 2 reference books or 10 normal books it would result in similar economies of scale.

However books can be used by multiple people over a period of time, the usual lending period of a book is 14 days, a book itself per year could be expected to be read 3-4 times depending on popularity (most of the books lifecycle is being lost or on the shelf) so if a book were to cost R500 its effective cost per year denoted by the amount of reads could be expected to be around R120.

That said I do believe that it is a still a bit expensive to afford a single laptop for every person in a school however it really is becoming more and more affordable

PS. just think how many tree's you will save if all these books were to have not been produced in the first place.