diminishing returns

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun plural A yield rate that after a certain point fails to increase proportionately to additional outlays of capital or investments of time and labor.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun idiomatic A condition in which additional inputs into an organization, project or process produce progressively fewer or lower-quality additional outputs, and may, in extreme cases, cause the total quantity or quality of outputs to decrease.