By Ian Bush

PHILADELPHIA (CBS) — It’s eye-watering: the cost of a revolutionary treatment developed in Philadelphia that’s designed to cure a rare type of blindness.

Wall Street, however, expected the price to be even higher:

“We think the value of Luxturna is in excess of a million dollars,” said Spark Therapeutics CEO Jeff Marrazzo.

Trendy ‘Raw Water’ Can Reportedly Give You Hepatitis, Other Diseases

Marrazzo says they’ll charge $850,000 — or $425,000 per eye afflicted with an inherited form of vision loss.

“We’ve tried to balance the concerns about accessibility that patients, families, and health care professionals have voiced; the concerns about budget impact we’ve heard from payers; as well as the need for us to build and create a path for a sustainable business model,” said Marrazzo.

Marrazzo tells CNBC the price for the gene therapy is outcomes-based — meaning patients get some of their money back if it’s not a success.

“I really saw in detail for the first time my mother and father’s faces,” he said.

Spotify Sued For $1.6 Billion Over Tom Petty, Doors Songs

Luxturna worked for Christian Guardino, who told KYW he received the single dose as part of a clinical trial at Childrens Hospital of Philadelphia with co-developer Penn Medicine.

“It’s just been incredible,” said Guardino.