Law of Diminishing Marginal Returns

You spent $1 million to market your new hair care formula, KnuckleSuckler, which makes hair from knuckles instead choose to grow on the head (mostly). From that first million dollar campaign, you gleaned $20 million in sales. Huge home run. But you wanted more more more.

So you spent $5 million more on marketing. And sales grew to $40 million. So that was good. Sales doubled. You had to spend $5 million to produce that $20 million more in marketing...but you had shareholders who also sang you the song more more more...and you spent another $10 million on marketing.

This time, you only grew sales by $10 million. Your easy money from marketing spend was clearly over, as you actually lost money on this last spend of $10 million to gain only $10 million in sales (which carried $6 million in costs). You've just shaken knuckle-hair-free hands with the law of diminishing marginal returns.

On the margin, your invested returns from your marketing spend diminished until they were zero or worse...and then you stopped. Thankfully.