Background: Persons with opioid use disorder (OUD) who are incarcerated are highly susceptible to opioid-overdose upon reentry. If initiated prior to release, extended-release naltrexone (XR-NTX) provides ~30 days of opioid-overdose protection in the community. XR-NTX's high cost is perceived as a barrier. Objectives: Estimate implementation and ongoing-management costs associated with different strategies of XR-NTX delivery to persons with OUD upon reentry. Methods: Data were from two multisite randomized-controlled effectiveness trials comparing pre-release XR-NTX + referral to community pharmacotherapy to: referral only (Study A); pre-release XR-NTX + post-release place-of-residence/mobile treatment (Study B). A micro-costing approach was used. We solicited estimates of resources required to deliver each strategy. All intervention-relevant resources were included and valued. The resource-costing method was used, with unit costs derived from sources reflecting national "real-world" costs. Resources varied by study, and included: labor, medication, supplies, and provider travel (mileage, time). Costs were categorized as fixed, time-dependent, and variable. Year 1 costs included (a), (b), and (c) variable. Subsequent annual costs included (b) and (c). Results: The in-prison XR-NTX process was estimated to take 2-3.5 hours. Study A adopted an in-house model. Study B's intervention was delivered by an outside team. Fixed/one-time costs and time-dependent costs were minimal; consequently, per-patient costs vary little with changes in patient caseload. Assuming full capacity, Year 1, per-patient costs were estimated as 979 dollars (Study A), and 3,458 dollars (Study B); subsequent annual costs were 976 dollars/patient and 3,453 dollars/patient, respectively. 1,320 dollars/patient in Study B was associated with travel to prison, and 1,007 dollars/patient was associated with the post-release mobile portion. Conclusions: Results are valuable to stakeholders interested in expanding XR-NTX OUD treatment in justice settings.