💸 How I Cut AWS EC2 S3 Data Transfer Costs to $0

😅 The Problem

At my previous company, in one of our AWS accounts, our AWS bill showed ~$300/month in “Data Transfer” even though all workloads were in the same region.

Our setup:

• EC2 batch jobs
• Uploading results/logs to Amazon S3
• Same region, no cross-region calls

I assumed:

“Same region = no cost.”

Wrong. 😬

🔍 The Root Cause

After checking Cost Explorer and VPC Flow Logs, I found the issue.
EC2 instances were talking to S3 using the public endpoint.

Example:

aws s3 sync /data s3://abccompany-logs/

That meant:

• Traffic left the VPC via NAT Gateway
• Crossed AWS’s public network
• Re-entered AWS to reach S3

So even though both were in the same region, we paid:

• NAT Gateway data processing: $0.045 per GB
• Regional data transfer charges for EC2 → S3 path

💰 At ~6 TB/month = $270+ in pure data movement.

Per AWS Docs:

“If data leaves a VPC through an Internet Gateway or NAT Gateway, standard data-processing charges apply.”

🧠 The Fix - VPC Gateway Endpoint for S3

The solution: keep the traffic inside AWS’s private network.

Here’s the Terraform snippet we used:

resource "aws_vpc_endpoint" "s3" {
  vpc_id          = aws_vpc.main.id
  service_name    = "com.amazonaws.eu-central-1.s3"
  route_table_ids = [aws_route_table.main.id]
}

Once created:

• EC2 → S3 traffic stays internal
• NAT Gateway no longer handles uploads
• Zero data-processing or transfer charges

Confirmed by AWS docs
:

“Gateway endpoints for Amazon S3 have no additional cost.”

💰 Results

After one billing cycle:

Metric                  Before  After
EC2 → S3 Transfer Cost    ~$300   $0
NAT Gateway Data    High    Minimal
Performance         Same    Same

🧩 Common Mistakes to Avoid

❌ Mistake                        ✅ Fix
Assuming “same region = free”       Verify network path
Using public S3 endpoint            Use Gateway Endpoint
Ignoring NAT data charges           Track via Cost Explorer
No flow visibility                  Enable VPC Flow Logs

🧠 Lessons Learned

This experience taught me that AWS cost optimization isn’t just reserved instances or savings plans, it’s about network awareness.

A single endpoint change saved us hundreds per month, with zero downtime or refactor.
It’s one of the simplest, most underrated optimizations you can make.

🪶 TL;DR

✅ EC2 → S3 via NAT = $$
✅ EC2 → S3 via VPC Endpoint = Free
✅ Check your data paths, even in the same region

Comments

[Ravavyr]

you could also just setup the S3 bucket as a mounted drive on your EC2 and cache all the assets using cloudfront and not get charged for using the NAT.

[Ishara Niwarthana]

But in our case, we heavily used S3 uploads (log files, reports, backups).

[leob]

Great advice - Jeff Bezos is already rich enough ;-)

[Ishara Niwarthana]

Exactly hehe