App Service Plans are priced based on the size and number of instances you run, and they are billed on a per second basis. For your case on the S2 plan, a single instance will cost $0.20/hour.
I see on your auto scale configuration that the minimum and default number of instances you will be running on this plan is two instances. With this, if the auto scale triggers are not hit, your App Service Plan would cost $0.40/hour.
With the configuration you shared this could run up to $0.80/hour, if maximum four instances are run after the auto scale triggers are met.
As App Service Plans are billed on a per second basis, the cost will be prorated on a per second basis for the number of instances you run.
For example:
if you were running two instances for 40 minutes, three instances for 10 minutes, and four instances for the last 10 minutes of an hour. The total cost of the App Service Plan for that hour would be roughly: $0.50 for that hour.
If you were to scale up or down your App Service Plan tier you can see more information about how this could affect pricing using this tool here: App Service Pricing
Thank you to cobethur for the assistance.