I have head this before, and I believe it to be true, but I don't know the source. Any ideas:
"Adding one more resource will not reduce effort by 50% but by only 75%?" Saving Changes...

Thought of supporting with Theory , but also you can create your Excel to calculate
What is the 'Law of Diminishing Marginal Returns'
The law of diminishing marginal returns is a law of economics that states an increasing number of new employees causes the marginal product of another employee to be smaller than the marginal product of the previous employee at some point.

Diminishing marginal returns states that in a production process, as one input variable is increased, there will be a point at which the marginal per unit output will start to decrease, holding all other factors constant. In other words, keeping all other factors constant, the additional output gained by another one unit increase of the input variable will eventually be smaller than the additional output gained by the previous increase in input variable. At that point, the diminishing marginal returns take effect.

For example, a factory employs workers to manufacture its product. As long as all other factors of production stay the same, at one point, each supplementary worker generates less output than the worker before him. Thus, each worker who follows provides smaller and smaller returns. If the factory continues to add new workers, it eventually becomes so cramped that additional workers hinder the efficiency of other employees, thereby decreasing the factory’s production. Saving Changes...

How to do it in Excel :
Spend (X) Return (Y)
500 475
1000 950
1500 1425
2000 1900
2500 2375

Using the same values in you Spend (X) column, and assuming that is column A, then, leave your present data in column B, and, in C2, enter: =0.95^(ROW()-1)*A2
and copy down.
Plot all three columns, selecting the Scatter(XY) graph. The resulting two curves are just what you want Saving Changes...