I have created 2 load balancers (for http and https) and they are connected to a backend storage. Now when I check the Infrastructure summary in the monitoring tab, I could see 2 different vms running. Do the cost for load balancer is related to these vms?
I have read the load balancer documentation and it was not clear how the LB's works internally.
It depends on what type of Load Balancer you're running, but basically it is all running and managed by the internal infrastructure of Google.
The HTTPS/HTTP are managed by the GFE and Andromeda which are the software-defined networks build by Google. You can read about this in the following documentation.
About the cost of the Load Balancer, it depends on the architecture of your environment. For this reason, I suggest you to check the following documentation which explains on detail the Load Balancer's pricing. In summary you will be charged depending on the traffic that goes trough the LB. This can be stimated using the GCP calculator.