It’s not really surprising that they spend around $12m a year to free on average 15 people a year. Anything involving the court system is expensive, I don’t know if the Innocence Project is run with lawyers working pro bono, but if not that right there is a big expense. Additionally they would need a staff of researchers to actually find persons who are good candidates to try and get freed. Then all the investigative work and research that has to be done on each candidates case, along with all the money spent testing each candidate’s case…$12m a year really seems pretty darn efficient actually.
It also isn’t surprising they free so few people. Of the 2.6m inmates the vast, overwhelming majority are factually guilty. Probably over 99%.
Of the ones who aren’t guilty, a subset of those can be demonstrably shown to have been improperly convicted and freed. So at least some portion of the innocent in prison, because of unique circumstances, probably cannot be freed by DNA testing or various legal appeals. Unfortunate, but it happens.
Of the innocent who can be freed, I’m sure the innocence project does its best to get as many of them in each year as it can. That being said, they aren’t perfect. They may only free 15 each year, but they probably try to free people who they fail to get freed (either because the DNA testing ends up proving the person was guilty or for some reason they just fail in their efforts) and those cases will cost probably just as much as the ones that result in releases from prison.