Community-based health insurance expansion has been proposed as a financing solution for the sizable informal sector in low-income settings. However, there is limited evidence of the administrative costs of such schemes. We assessed annual facility and district-level costs of running the Community Health Fund (CHF), a voluntary health insurance scheme for the informal sector in a rural and an urban district from the same region in Tanzania. Information on resource use, CHF membership and revenue was obtained from district managers and health workers from two facilities in each district. The administrative cost per CHF member household and the cost to revenue ratio were estimated. Revenue collection was the most costly activity at facility level (78% of total costs), followed by stewardship and management (13%) and pooling of funds (10%). Stewardship and management was the main activity at district level. The administration cost per CHF member household ranged from USD 3.33 to USD 12.12 per year. The cost to revenue ratio ranged from 50% to 364%. The cost of administering the CHF was high relative to revenue generated. Similar studies from other settings should be encouraged.