The disease itself is incredibly expensive. The price of breakthrough treatments will come down faster than the costs of having the disease.
[^1] see FDA press release and monograph. addditional doses won't do much because by then the immune system has learned to attack the AAV vector -- a different vector strain would be needed and that would be a new drug to be approved
[^2] no idea what the turnover rate is. and honestly, the FDA isn't even sure it helps with anything, hence conditional.