AWS data transfer costs

Some times instance reside in different AZs and these instances need to share data. Instances also use S3 for things like backups. Here is breakdown of data transfer cost for some typical scenarios: 

1. Instance to Instance in same AZ 
Cost: Free
2. Instance to Instance in different AZ
Cost: Inter-AZ charge for both inbound and outbound
3. Instance to Instance in different account in same or different AZ
Cost: Inter-az charges are applicable for data transfer charges between instances in the different AZ's (same region) and in different accounts
4. Instance to S3 in the same region
Cost: Free