When the government says a new drug is approved for sale, the first thing many companies do is announce what it will cost. Not Spark Therapeutics, though.

The gene-therapy pioneer, based in Philadelphia, won approval on Tuesday for a gene replacement that, as we have written about before, helps people with a rare form of blindness. The treatment, called Luxturna, is the first gene therapy the U.S. Food and Drug Administration has approved to treat an inherited genetic disorder, and the third gene treatment this year (the other two were for cancer).

The agency’s administrator, Scott Gottlieb, said, “We’re at a turning point when it comes to this novel form of the therapy.”

We’re also in new territory when it comes to gene-therapy prices. So we can guess at why Spark didn’t release theirs: the cost of this treatment is likely to be mind-boggling. Maybe as much as $1 million per person.

Put it this way: the average American family would take 18 years to buy a single dose of the eyesight-saving drug, even if they bought nothing else and paid no taxes.

The problem for Spark is that only about 10 Americans per year will even need this drug. With such a small potential market, there’s almost no price that makes it very profitable.

Spark said Tuesday that it won’t be releasing the price until early next year. The Foundation Fighting Blindness says Spark has told it that the company has been trying to "educate" insurers “as to the value of the treatment” and is committed to getting the treatment to patients who need it.