Oh man. I agree with your sentiment, but this is a remarkable over simplification!
The sequence may cost 700$ to synthesise. But it's delivery vector is a virus. Synthesizing the DNA, encapsulating the DNA in the virus.. not trivial stuff.
While I agree that drug prices in the US are absurd beyond comprehension or justification, and I've had a ring side view of life sci research, drug development as well as the drug pricing; to posit that, hah, a path breaking gene therapy product just costs a few hundreds or a couple of thousands to produce is misrepresenting the complexity as well as the significance of the achievement.
The amount of research (largely public funded but for the clinical trials, which may also have been public funded: no idea) to get to this point is staggering. the company also likely invested large amounts to Shepard a lab possibility into a viable treatment. FDA approval is a big deal because they are, justifiably, exceedingly strict. It is an arduous process.
There has to be a better solution, which would start with allowing the US CMS / Medicare & Medicaid to negotiate drug prices with the manufacturers. The problem is policy.