A few years ago, Qantas did the sums on fitting them and came to the conclusion that the aircraft would have to operate for 10 years before the savings offset the installation cost. Also, the savings achieved from winglets is greater when the average sector length is longer - Qantas has been using the majority of their B767's on short domestic sectors for some years now whereas the Air NZ B767 fleet flys longer international sectors thus making the installation of winglets more viable.
The company that manufactures the blended winglets may have reduced the price since QF did the sums in order to stimulate demand.
The second batch of QF B738's (VH-VY_ series) were originally going to be delivered without winglets because of their huge cost, but when the price was discounted, Qantas changed the order to include them.