I commented this elsewhere, but to answer your question,
Zolgemsma is a modified version of adeno associated virus and has to be grown under specific conditions. It costs $500k-$1m per production. It’s also a one time injection that functionally cures the person of the disease. There are a couple other options but for comparison, the other therapeutic is Spinraza which is an intermittent intrathecal infusion which is $805,000 for the first year of therapy and $380,000 per year thereafter for the rest of your life.
SMA type I also is 100% fatal by year 2-3 and the baby dies without being able to even lift their head. It’s a terrible prognosis.
To be clear, I think we should bear the actual costs of research, development, and manufacture as a society and not profiteer off the sick, but there are some contributory reasons for the price.