Billing of my GCP instance goes down every week and increases at the start of every month Why is this fluctauation

Question

I'm running on Google Cloud. I was looking at the costs and I noticed something weird - every week the daily cost of running machines is going down, and then on the 1st day of the month, it goes back up.

Why exactly does this happen? I can see a significant difference in the pricing.

Answer 1

The prices don't change, but an increasing discount is applied when your instances run longer.

Google Compute offers a sustained use discount for running specific Compute Engine resources a significant portion of the billing month. The discounts come in a couple of tiers and start to apply after running first 25% of a month, then increase after running 50% and 75% of the month. 

See https://cloud.google.com/compute/docs/sustained-use-discounts