Data Transfer Gotcha Updates

README.md

@@ -2378,6 +2378,8 @@ Billing and Cost Management
 -	Some of the most common gotchas:
 	-	🔸*AZ-to-AZ traffic:* Note EC2 traffic between AZs is effectively the same as between regions. For example, deploying a Cassandra cluster across AZs is helpful for [high availability](#high-availability), but can hurt on network costs.
 	-	🔸*Using public IPs when not necessary:* If you use an Elastic IP or public IP address of an EC2 instance, you will incur network costs, even if it is accessed locally within the AZ.
+	-	🔸*Managed NAT Gateway data processing:* Managed NAT Gateways are used to let traffic egress from private subnets--at a cost of 4.5¢ as a data processing fee layered on top of data transfer pricing. Past a certain point, running your own NAT instances becomes far more cost effective.
+	-	🔸*Some services do cross-AZ traffic for free:* Many AWS services you'd not consider on their own merits offer a hidden value of free cross-AZ data transfer. EFS, RDS, MSK, and others are examples of this.
 -	This figure gives an overview:
 
 ![AWS Data Transfer Costs](figures/aws-data-transfer-costs.png)