the area behind the demand curve between the two prices measures the value to consumers of being able to purchase avocados at the lower price. Provided the planner can estimate the shape of the demand curve, the project's benefit can be measured.
If the supply curve of the commodity under consideration is upward sloping, then changes in producer surplus (also explained in the appendix at the end of the book) can be brought into play. For example, in the cost-benefit analysis of rent controls, the change in landlords' surplus could be estimated given information on the shape of the supply curve of rental housing.