I'm unsure now, I tried to find the article I read that from. Obviously its super hard to get the true material cost of a drug, I dont think Biogen wants that information out there.

However I did find a light description of their process

In common with the production other viral vectors, the vectors used to make Zolgensma are produced in adherent cell lines. Working with adherent cell lines requires specialist technologies and systems. For example, stirred tank bioreactors are better suited to the growth of cells in suspension rather than those that are fixed. Production of a single Zolgensma batch takes 30-days Althoff told us, explaining that after cell expansion, triple transfection of HEK293 cells is used to make the adeno associated virus (AAV) vectors. Downstream processes consist of a series of capture and filtration steps used to achieve the desired impurity profile and full/empty capsid ratio all of which require specialist technologies. “We complete the entire process in-house” Althoff said, adding “The process takes approximately 30 days start to finish. Product can be stored for 12 months.”

Based on this I found an article that breaks down the cost of AAV vector production: https://www.insights.bio/immuno-oncology-insights/journal/article/15/cost-modelling-comparison-of-adherent-multi-trays-with-suspension-and-fixed-bed-bioreactors-for-the-manufacturing-of-gene-therapy-products

From figure 5 most doses are around 10K$ considering all costs, now we can be super generous and say that zoglmas material cost is something from 10k$ - 100K$ per dose.