How do we compare the cost of running MongoDB in GCE vs using Google Cloud Datastore?

[Wajahath via StackOverflow]

I knew MongoDB and Google Cloud Datastore offer NoSQL database systems.
I'm new to deploying a database. These are some of my confusions:

1. How do we compare the cost of running MongoDB in Google Compute Engine vs. using Google Cloud Datastore? Can you quote a small example estimate? We may find many related articles. But I couldn't find one that address this specific question.
2. I came to know about the Click to Deploy option in Compute Engine. What is the difference(in cost/performance) between manually configuring the Compute Engine for MongoDB and Click to Deploy option?
3. Within the Click to Deploy option you'll find two options(this and this) we can choose for MongoDB. Can you spot the difference between them? There is also a significant price difference between them.
4. Is it worthy to start developing in Google Cloud Datastore leaving my MongoDB skills?

I would prefer to know the answers on the basis of cost, development overhead and perfomance.

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