"A case study of the island of Rhodes [in 1993] estimated that the cost of planned projects for protecting the marine environment – primarily for sewage treatment and disposal – amounted to US$ 61 million. The benefits were estimated to be US$ 152 million, almost two and a half times greater than the cost. Similarly, a study of Izmir Bay, Turkey [in 1993] which was concerned primarily with sewage, but also, to some extent, with industrial pollution – estimated the discounted cost of controls between 1988 and 2025 to be US$ 1.3 billion, while the discounted benefits were in the range of US$ 4.77-10.2 billion (not including multiplier effects). Thus, benefits in this case would exceed costs by a factor of 3.6-7.8. (The range reflects different assumptions about the future of the tourism industry.)
Investment in sewage treatment and disposal clearly shows considerable benefits. Its feasibility depends on a variety of factors; among the most critical of them, in many situations, is the willingness of responsible authorities to adopt appropriate technical and financing solutions – coupled with external technical and financial assistance in the poorest countries."
Source: GESAMP71:100
GESAMP (IMO/FAO/UNESCO-IOC/WMO/WHO/IAEA/ UN/UNEP Joint Group of Experts on the Scientific Aspects of Marine Environmental Protection). 2001. Protecting the oceans from land-based activities - Land-based sources and activities affecting the quality and uses of the marine, coastal and associated freshwater environment. Rep. Stud. GESAMP No. 71, 162 pp. ISBN 82-7701-011-7.