Interactive NAT, endpoint, peering, TGW, and PrivateLink economics

List egress by subnet and Availability Zone, destination service, protocol, monthly bytes, connection count, and failure requirement. Private instances reaching the internet need a translation path, but traffic to supported AWS services may use a gateway or interface endpoint instead. An S3 gateway endpoint can remove high-volume S3 traffic from NAT while leaving bucket and endpoint authorization as separate checks. Interface endpoints create private ENIs for supported services and introduce endpoint-hour and data-processing dimensions, often per AZ. Centralizing endpoints or egress reduces fixed resources but can add Transit Gateway processing, cross-AZ transfer, DNS complexity, and a wider failure dependency. Route each subnet deliberately; a NAT gateway in another AZ may work normally yet violate an AZ-failure requirement and create extra traffic legs. The first cost artifact should separate internet egress, AWS service access, and private provider service access because each has a different valid topology.

A NAT gateway is managed by AWS and provides elastic service capacity within its documented model, reducing instance patching, sizing, and failover work. It has gateway-hour and data-processing charges, and resilient high-volume designs commonly use one per AZ with same-AZ routes after endpoint traffic is removed. A NAT instance is an EC2 instance configured for translation. It can be less expensive for low-volume noncritical development, but the customer owns AMI patching, instance sizing, bandwidth, security-group configuration, source/destination check handling, monitoring, scaling, and failover. Its availability follows the instance and architecture built around it. AWS's direct NAT comparison is the claim source for these distinctions. One shared NAT gateway can beat several gateways when traffic is tiny and the business accepts the cross-AZ path and outage dependency. Per-AZ gateways reverse the choice as volume, locality, and AZ survival dominate. A NAT instance reverses it only when operational burden and lower availability are explicitly acceptable; it is not a free equivalent.

Peering directly connects two VPCs and is nontransitive, making it suitable for a small number of explicit relationships. As networks grow, route tables and pairwise operations can dominate its low service overhead. Transit Gateway supplies a routed hub with attachment and processing cost, earning that cost when many VPCs and on-premises networks need transitivity or centralized controls. PrivateLink exposes a selected provider service through endpoints rather than creating general bidirectional routing. It limits reachability and CIDR coupling but is wrong when arbitrary network communication is required. Compare fixed endpoints or attachments, variable processing, cross-AZ and cross-Region legs, DNS operations, route ownership, and failure rerouting. Test the return path as rigorously as the forward path. A private endpoint does not bypass endpoint policy, resource policy, or security-group controls. As a topology changes from two VPCs to dozens, or from broad routing to one service, rerun the candidate set instead of extending the original design indefinitely. Operational simplicity has a measurable cost value when it reduces route errors and outage duration.