Upon a time, back in 2010, I made an interesting POC, when REST started to become a stronger "de facto" performer in the industry.
In that time of my life, I can say that I had more free time to invest in things that I liked, or even chance to create crazy POCs for demonstrating the potential from my employer from that time.
Well, in early 2010, I create a kind of a prototype of an Web App that could act a backend to facilitate people to create their "WebServices" (that time we did not mentioned APIs that much).
That was a prototype made with my brother (Jefferson), when I was helping him in one of his efforts to create some cool stuff on Google App Engine on that time.
That prototype is still into my GitHub account, more precisely here: https://github.com/edgars/corerest/tree/master/corerest/
Here some screenshots of that "prototype":
CoreRest version 1:
Nowadays check it out how big is the API Manager business and market, well, no regret on that, but sometimes we have some cool ideas, and we are not able to believe in ourselves, that is the best lesson that no book, mba, training, mentor, related courses or any related to entrepreneurship can't give us: "The capacity to believe that we have the potential to make the things happens"!
The answer for how much cost an idea that you never executed except on papers or slides…. I can't tell how much you could earn, but for sure you missed a good chance for at least to learn new things and who knows could get able to share some nice histories as well :)