Festival Park - diminishing returns illustrated

I made this diagram a few months ago to illustrate the concept of diminishing returns as applied to Festival Park. I never got around to posting it. Diminishing returns is a basic concept in economics. The idea is that the first million dollars you spend will have the most impact, and then each additional million gives you less and less. At some point, it's no longer worth putting money into a project because the return on your investment is so little.

My thought is that Festival Park is good enough and the extra $5M or whatever they're spending is at the point in the project where the return is not worth the investment.