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.
zolgensma is the second gene therapy ever approved in the US. The technical risks are very high and making them at scale at a high level of quality required rare and expensive expertisr