The cost of sustainable infiltration and inflow (I/I) reduction is dependent on the sources of I/I and the methods used to rehabilitate the sewer collection system. For facilities planning it is desirable to estimate the cost of I/I reduction efforts to determine if these efforts are cost effective compared to other means of managing wet weather peak flows. Often at the facilities planning level, a source identification program or sewer system evaluation survey has not yet been performed so it is difficult to know the information needed for traditional approaches to cost estimating. This paper presents an alternative approach to estimating the cost of I/I reduction that is not technology dependent because it is based on performance results from a wide range of I/I reduction projects. The Water Environment Research Foundation (WERF 2003) documented demonstration projects; the study reports the I/I reduction achieved and the cost. These projects along with other available I/I project experiences from Milwaukee, Wisconsin and King County, Washington were used to create a performance based cost function for the unit cost of I/I removed based on pre-rehabilitation I/I rates. Facilities planning efforts often compare the cost of major system upgrades (such as additional treatment and storage capacity) to the cost of I/I reduction to achieve the desired system performance goals. This method is a means of estimating the cost of I/I reduction which can be used for planning. The advantage of this approach is that specific details and methods of I/I rehabilitation do not need to be known (as is commonly the case at the facilities planning level) to proceed with a meaningful estimate of the relative cost effectiveness of I/I reduction efforts. An example, based on experience from the Milwaukee Metropolitan Sewerage District, shows how the approach has been applied to determine the cost of I/I reduction for sustainable facilities planning. The performance based approach has also been used to estimate the average annual cost of sustained I/I maintenance to keep I/I rates constant over the life of the infrastructure. On a smaller scale, this same approach can (and has been) applied to specific interceptor relief projects to determine if I/I reduction may be more cost effective than increasing the conveyance capacity. Therefore, this method can be applied to overall system master planning as well as preliminary planning of smaller conveyance upgrades projects.