How can I reduce App Engine billing cost?
I think your cheapest configuration is just setting max_instances: 1 and commenting out the other options.
When you have traffic, the maximum number of instances that you will have will be 1. When there's no traffic, your instance goes down (effectively 0).
The downside with this approach (not having min_idle_instance as you currently do) is that brand new traffic to your site will take some time because of the time for your instance to be started.