I think you have the motivation wrong. The cost-benefit economics work if you want to have fun building something, learn a lot and share it with others. It doesn't work if the goal is to make up to $100/month selling ads; getting a part-time job would be a better path. In this scenario not finishing your side project is the correct decision, and not starting an optimization of that.
To me, trying to make money with random projects is the most motivating thing. A dollar earned from some little project is emotionally to me worth many times that of the same dollar I'd earn as salary (as long as I don't starve). Most of my friends do not seem to share this feeling.
Also the internet is very big. You can sometimes have success with something, even it's a very silly badly implemented little thing. What people like, how you happen to get traffic, it's all quite unpredictable.
