You could talk about diminishing returns.
This two-word phrase has two related meanings. One is a more formal economic meaning, where further investment in an enterprise is unlikely to produce proportional dividends.
In a more informal sense, the expression simply refers to what you mentioned in your question: there's too little to gain from the effort, to the point where it's not worth the effort.
The M-W dictionary alludes to both of those meanings with these definitions, even using the word effort in its definition:
: a rate of yield that beyond a certain point fails to increase in proportion to additional investments of labor or capital
: benefits that beyond a certain point fail to increase in proportion to extended efforts
Wiktionary mentions that the phrase is idiomatic, while the Phrase Finder gives this note:
In common usage, the "point of diminishing returns" is a supposed point at which additional effort or investment in a given endeavor will not yield correspondingly increasing results. So, when you have reached that point, it's better to give up the endeavor.
Collins lists this example sentence, showing the phrase being used by a Glasgow newspaper in its more informal sense:
By becoming a pop diva again, she has revitalised a career that, in the mid-1990s, was an exercise in diminishing returns.