Answer:
It is difficult to estimate the marginal social benefits of supplying a public good.
Step-by-step explanation:
A public good will always be beneficial to the society. However, the exact social benefits it brings cannot be exactly (quantitatively) measured. The costs behind the creation of such a good can be measured (e.g. How much of funding is needed to build a hospital in a village?).
On the other hand, the marginal social benefit for each unit of resources invested cannot be identified (e.g. How many people would benefit for a million dollars invested?).