In order to get the traffic usage information per VM instance, you can check the 'Monitoring' tab under 'VM instance details' page which will provide incoming/outgoing network bytes up to last 30 days.
However, if you would like to record network flows sent from and received by VM instances, the best approach would be to use VPC Flow Logs feature. If enabled for a subnet, VPC flow logs collects data from all VM instances in that subnet. Also, you can filter the flow logs by VMs and by applications to understand traffic changes. Furthermore, you can find the destination of the outgoing traffic based on continent, country, region, city etc. More information about using VPC Flow Logs can be found in this help center article it also include the pricing for VPC Flow Logs.
In you last inquiry about charges for traffic you can check this Google Cloud Platform Pricing Calculator.