FORTUNE — The difference between Ronald Washington spending the rest of his life in prison without the possibility of parole, or not, came down to $21. Washington stood accused of stealing two Michael Jordan jerseys, which retailed for $60 each. His lawyer argued that at the time of the theft the jerseys were on sale for $45 apiece. The drop in price was crucial: if the value of the goods Washington stole totaled less than $100, the crime was a petty theft; over $100, it stood as a felony.

When the judge read the verdict—guilty, for the felony—”I felt as though somebody had just taken the life out of my body,” he wrote to the ACLU, which last week issued an astonishing report about the 3,278 prisoners who, like Washington, are serving out life sentences for nonviolent crimes.

It is not usually this easy to peg an exact number to the cost of injustice, but in the bizarre, draconian world of mandatory sentencing, it is. Ronald Washington now sits in a cell in Angola, living his life with no chance of reentering the workforce to become a taxpaying citizen. So for a $21 discrepancy, the state will spend an estimated $500,000 keeping Washington incarcerated for the rest of his life.

By the ACLU’s most conservative estimates, taxpayers lose $1.15 billion to lock up for life individuals who were, for example, caught sharing LSD at a Grateful Dead concert, or stole tools from a shed, or carried drugs for an abusive boyfriend. In Louisiana—a state that spends about $183 million on its nonviolent offenders serving life—91% of all those incarcerated in such cases are black.

Creating laws that require mandatory life-sentences for nonviolent crimes does not simply pervert the notion of justice; it is the most expensive way possible to dole out punishment. There is no humanitarian or economic sense to such a practice.