For those who are interested to know how much App Engine Standard Environment roughly will cost in real world applications here is my simple formula for it:

Average RPS * $15

So for 100 RPS for one month it costs around $1,500.

Also, you can optimize for cost by adding the following to your app.yaml:

threadsafe: true

Also Golang and Node costs less than other environments.

My two cents.

Update: Formatting.