The Housing and Health study examines the effects of permanent supportive housing for homeless and unstably housed persons living with HIV. While promising as an HIV prevention intervention, providing housing may be more expensive to deliver than some other HIV prevention services. Economic evaluation is needed to determine if investment in permanent supportive housing would be cost-saving or cost-effective. Here we ask -- what is the per client cost of delivering the intervention, and how many HIV transmissions have to be averted in order to exceed the threshold needed to claim cost-savings or cost-effectiveness to society? Standard methods of cost and threshold analysis were employed. Payor perspective costs range from $9,256 to $11,651 per client per year; societal perspective costs range from $10,048 to $14,032 per client per year. Considering that averting a new case of HIV saves an estimated $221,365 in treatment costs, the average cost-saving threshold across the three study cities is 0.0555. Expressed another way, if just one out of every 19 Housing & Health intervention clients avoided HIV transmission to an HIV seronegative partner the intervention would be cost-saving. The intervention would be cost-effective if it prevented just one HIV transmission for every 64 clients served.