A 2010 jetties study from the legislature found the cost of building jetties from $3.5 million and $10 million, and that up to another $2 million would be spent every year for maintenance that included replacing sand washed away from the backside of the structures. The study, however, also found that over 30 years the economic benefits of the protective structures ranged from $27 million at Ocean Isle to over $320 million at Bald Head Island.
Ignoring the obvious questions behind the benefit numbers ... discounting the annual costs at 3% over 30 years, taking random numbers from a uniform distribution of the variable costs and benefits, then the average net present value of a jetty is about $200 million.
Ok, not ignoring the obvious questions about the benefits ... $320 million at Bald Head Island? That is about $3.2 million in benefits for each resident household over 30 years. That number seems unrealistically high. Discounting at 3% over 30 years, it is about a $165,000 annual household benefit.
My Google search didn't find anything about the study except this summary: http://dcm2.enr.state.nc.us/CRC/tgs/finalrecs.pdf.