You can use Google Cloud Platform Pricing Calculator for that or calculate it yourself using the prices in App Engine Pricing:
- vCPU per core hour $0.0526
 - Memory per GB hour $0.0071
 
So in your case:
- 1 vCPU * 0.0526 $ per vCPU = 0.0526 $ per hour
 - (0.6 GB + 0.4 GB) * 0.0071 $ per GB = 0.00426 $ per hour
 
Then 0.05686$ per hour for 240 hours is 13.6464$ for one instance over 10 days, which is close to what you paid.
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solved Google App Engine flexible price