I work doing safety testing for drugs just like this. Setting aside all the money it takes to develop one of these viral gene therapy drugs, the cost to manufacture a single dose on a lot of them is somewhere around 50-90,000 dollars. The viruses need to be grown, then emptied of their regular genetic material, have the new, specially designed genetics put in, then replicated up to a level that can be infused into a person. Every single one of those steps requires highly skilled people to ensure it happens correctly. Growth of viruses need to happen in cell tissue culture, which also costs a lot for all the materials to do correctly, then all the supplies needed to purify out the virus from all the other crud in manufacturing. Then, once the product is made, it still has to be quality tested, to ensure things like dosage is correct, and that there isn’t anything left that’s contaminating. And all this doesn’t even count the years and years of research and development that goes into getting a virus with the correct components to be effective at treatment but otherwise non damaging to a patient. Drug product patents have to be filed pretty much as soon as a compound is discovered, so well before clinical trials even begin. They’re often only good for 20-25 years. So, if you figure 7-10 years of base research and development, another 3-5 years of clinical trials, getting FDA approval, etc, on average a company only has 3-5 years to make back ALL the money they put into a new drug, plus some profit to keep the doors open, lights on and investors interested in funding the next project.