Google Cloud Platform's Pricing Calculator shows that the specs in your app.yaml turn out to be Total Estimated Cost: $41.91 per 1 month so your costs seem right.

AppEngine Flexible instances are charged for their resources by hour. With manual_scaling option set your instance is up all the time, even when there is no traffic and it is not doing any work. So, not turning your instance down during the idle time is the reason for the $40 bill. You might want to look into using Automatic or Basic scaling to minimize the time your instance is running, which will likely reduce your bill considering you don't have traffic 24/7 (you will find examples of proper app.yaml settings via the link).

Note that with automatic/basic scaling you get to select instance classes with less than 1 dedicated core (i.e. 0.2 & 0.5 CPUs). Not sure if setting CPU to be > 0 and < 1 with manual_scaling here would also work, you might give want to give it a try as well.

Also, don't forget to have a detailed look at your bills to see what else you are potentially being charged for.

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After few searches, that seems to be the lowest possible configurations. See related answer here:

Can you use fractional vCPUs with GAE Flexible Environment?

At least for now, there is no shared CPUs so you'll pay for one even if your app is using an average 2% of it. Maybe adding few star here will help changing that in a near future:

https://issuetracker.google.com/issues/62011060

google app engine tutorial, Download the App Engine SDK and set up your App Engine environment. Authenticating Users. Integrate your guestbook application with Google user accounts using the Users service. Handling User Input Download the App Engine SDK and set up your App Engine environment. Authenticating Users. Integrate your guestbook application with Google user accounts using the Users service. Handling User Input

After reading articles on the internet I have created 1 f1-micro (1 vCPU, 0.6 GB memory) VM instance of bitnami MEAN stack which costs ~$5.5/month. I was able to host 1 Mongo DB instance and 2 Node.JS web applications in it. Both the applications have different domain names.

I have implemented reverse proxy using Apache HTTP server to route traffic to appropriate Node.JS application by it domain-name/hostname. I have documented the steps I followed here: https://medium.com/@prasadkothavale/host-multiple-web-applications-on-single-google-compute-engine-instance-using-apache-reverse-proxy-c8d4fbaf5fe0

Feel free to suggest if you have any other ways to implement this scenario.

google app engine architecture, Quickly reach customers and end users by deploying web apps on App Engine. With zero-config deployments and zero server management, App Engine allows you to focus on writing code. Plus, App Engine Quickly reach customers and end users by deploying web apps on App Engine. With zero-config deployments and zero server management, App Engine allows you to focus on writing code. Plus, App Engine

The cheapest way to host a Node JS application is through Google Compute Engine, not Google App Engine. This is because you can host it for 100% free on Compute Engine! I have many Node apps that have been running for the last 2 years, and I have been charged a maximum of a few cents per month, if any at all. As long as you are fine with a low spec machine (shared vCPU) and no scaling, look into the Compute Engine Always Free options. https://cloud.google.com/free/docs/always-free-usage-limits#compute_name

The only downside is that you have to set up the server (installing Node, setting up firewalls etc). But it is a one time job, and easily repeatable after you have done it once.

google app engine pricing, Pricing for related Google Cloud products. App Engine uses Cloud Build to build and deploy apps. Cloud Build has its own quotas, and each time you deploy your app to App Engine you use some of this quota. Cloud Build provides a free quota, so you won't incur costs for deploying App Engine apps until you surpass the free quota. Pricing for related Google Cloud products. App Engine uses Cloud Build to build and deploy apps. Cloud Build has its own quotas, and each time you deploy your app to App Engine you use some of this quota. Cloud Build provides a free quota, so you won't incur costs for deploying App Engine apps until you surpass the free quota.

Use the Basic, not Flexible. It is a better fit and far cheaper for you.

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