There is also the point John has mentioned on whether you can say that an environmentally designated area is worth more than a non-designated area? There are methods such as contingent valuation, and Clawson's method which deal with how much people perceive areas are worth e.g. by asking how much they would pay to use a beach. There are obviously major limitations with this, and many people believe that putting prices on such items is completely ridiculous anyway.
Also, it should be noted that even though some sustainable methods may not work that well, hard defences have a relatively short life-span with a much larger cost. They also harm coastal environments by starving coastal sediments down shore. In my opinion, Local Planning Authorities will often use such hard defences to solve the perceived problem rather than the problem at hand. As long as the people perceive the problem is being dealt with, through observing a large sea wall being constructed for instance, then security will have been established through the local community. This is well understood and used notion used by LPAs to maintain their positions in Council. (Michael Clark, 1998).