Genentech's Ed Lang: “The monthly cost of Kadcyla in the United States is $9,800. The estimated cost of a course of Kadcyla is approximately $94,000. This is based on people taking Kadcyla for 9.6 months. When pricing Kadcyla, we considered a variety of factors including that cost of the combination of Tykerb and Xeloda (approximately $10,460 per month) which is the current standard treatment in this line of therapy and the comparator in the pivotal EMILIA study.
I.e. monthly Kadcyla price doubles that of Herceptin ($9,800 vs. $4,500) + prolongs survivval in mBC (lived a median of 5.8 months longer [overall survival] than Tykerb + Xeloda in 2nd line mBC) + is better tolerated than Tykerb + Xeloda as standard of care in 2nd line mBC (i.e. less drug discontinuation)
As a reference, Herceptin sales in 1st line mBC are $2bn and no sales in 3rd line:
pricing and treatment duration in mBC alone make Kadcyla a USD4bn drug