"In 2003 among facilities operated by the
Federal Bureau of Prisons, it cost $25,327 per inmate." you have to assume that number has gone up in the last 12 years.
I guess it depends on what you consider "saving future lives" think of all the high risk criminals we have in prisons, and then think about how for each one the government could put $20k-$30k per criminal A YEAR to better use.
2010 numbers show 166,700 people were in prison for murder. if you take the average cost from 2003 that comes to 4,167,500,000 for the entirety of the united states.
I don't know about you, but I can think of a whole lot of ways to save peoples live, or make the lives of those who don't murder people, better with 4 billion dollars a year.