This article presents the evolution of the cost of detention in the canton of Vaud from 1845 to 2015. It demonstrates that jails are far from being unbearably expensive for the public finances. Actually, the work of prisoners has represented a significant source of income for the state, through the sale of what they have produced. Over certain periods, carceral institutions were even able to almost cover the total amount of their expense, thanks to prisoners’ work. During the second half of the 20th century, in spite of growing detention costs – caused among other factors by the increasing wages of prison staff – the prisons’ budget remains low by comparison with the Gross Domestic Product. This analysis questions widespread preconceptions concerning the high costs of detention, prone to kindle instrumental political points.