Congressional Budget Office Cost Estimate: S. 312, COVID-19 Safer Detention Act of 2021 [open pdf - 325KB]
From the Document: "'The bill would'[:]  Allow federal prisoners sentenced before November 1, 1987, to apply for compassionate release[;] [and 2] Shorten certain elderly prisoners' sentences by applying credits for good conduct time to eligibility for early release[.] 'Estimated budgetary effects would mainly stem from'[:]  Reductions in discretionary spending by the federal Bureau of Prisons because some prisoners would be released earlier than under current law[;] [and 2] Increases in direct spending for health care, Social Security, and other federal benefits[.] 'Areas of significant uncertainty include'[:]  Projecting the number of elderly incarcerated prisoners and the number who would be eligible for release from prison[;]  Projecting whether prisoners would be confined in residential reentry centers or at home, and the difference in costs between those options compared with confinement in federal prisons[;] [and 3] Estimating the number of offenders released under the act who would receive federal benefits[.]"
Congressional Budget Office: http://www.cbo.gov/