Of course the follow up question is there any way to get that number lower.
Per the financial statements, ~$500M of that $700M was r&d (the rest was general and administrative expense). A lot of the r&d spend was payments to third party research and manufacturing firms. Given the small clinical trial size I'd imagine more money went to contract manufacturing than research.
There are very few companies and people that can make gene therapies at scale at acceptable quality levels. It seems like this expertise is quite expensive. There was probably a lot of process development here since making these drugs at scale is so new (here "at scale" means "enough for ~100 patients")
They also spent a lot building out their own manufacturing site.
They also had a $135M license payment, I think to the company they licensed their viral vector from
So something like they spent $100M on this one winning idea but also $600M on 50 other ideas that went nowhere.