Every month, Telita Hayes adds nearly $200 to the commissary account for her ex-husband, William Reese, who has been in the Louisiana State Penitentiary for 28 years.

Each prisoner there is given three meals a day and some personal hygiene items, like soap and toothpaste. But when Mr. Reese gets hungry between meals, or when his state-issued supplies run out, the commissary money buys him extra food and other necessities.

That is not the only way his imprisonment drains her wallet. On top of the $2,161 she has put in his commissary account so far this year, Ms. Hayes has paid $3,586 in charges for talking to him on the phone when she cannot make the hourlong drive to the prison, and even $419 for emails sent through the prison’s email system.

The Bureau of Justice Statistics estimates that the United States spends more than $80 billion each year to keep roughly 2.3 million people behind bars. Many experts say that figure is a gross underestimate, though, because it leaves out myriad hidden costs that are often borne by prisoners and their loved ones, with women overwhelmingly shouldering the financial burden. These costs rise during the holiday season, relatives say, as they make more visits, call more often and send more care packages.