1) much more sophisticated/cutting edge in terms of R&D,
2) the R&D is all of a new kind, so there has been little prior amortization of resources from prior successes.
3) _curative_, so from a purely market perspective a XX% discount on a [cost/year * lifetime] relative to a small-molecule drug that needs to be taken regularly, forever, is in these very high ranges, and still economically appealing from a cold hard actuarial perspective.
The high prices also attract advanced technology development in the area, and I think society wants & demands that rapid technological development of these powerful tools. Imagine trying to pay competitive salaries to a large number of sophisticated genetic engineers and all of their tools and equipment, when there are literally only a five or six products on the market with which to support their efforts.
It is still very unclear how we as a society should be paying for that development. And placing that burden on (those) individuals seems unreasonable.