The True Monthly AWS Cost of Running a 100K-MAU B2B SaaS in 2026: An Empirical Breakdown

Reading a Real AWS Bill

The published AWS pricing pages are correct but almost never predictive — the actual bill is dominated by line items that don't have a marketing page. This is a de-identified breakdown of the actual monthly AWS spend for a well-architected B2B SaaS running around 100,000 monthly active users in 2026.

The Workload

• 100K MAU, ~35K DAU peak
• Node.js / Next.js on ECS Fargate + a small Lambda footprint for webhooks
• RDS Postgres 15 primary + one read replica, Aurora-compatible
• ~1.2TB of user-uploaded files stored in S3, served via CloudFront
• Datadog for APM and logs
• Multi-AZ, single region (us-east-1)

Line-Item Monthly Bill (2026 prices)

| Category | Line item | Typical cost |

|---|---|---|

| Compute | ECS Fargate — 6 tasks, 1 vCPU / 2GB, 24×7 | $340 |

| Compute | Lambda invocations (~15M/mo) | $80 |

| Compute | Application Load Balancer + rules | $28 |

| Database | RDS Postgres primary db.m6g.xlarge Multi-AZ | $780 |

| Database | RDS read replica db.m6g.large | $270 |

| Database | Automated backup storage (~180GB) | $18 |

| Storage | S3 Standard, 1.2TB stored | $28 |

| Storage | S3 PUT/GET/LIST requests (~2M/day mixed) | $65 |

| Storage | S3 lifecycle transitions to IA (~40%) | -$8 |

| Network | CloudFront egress ~3.5TB/mo | $290 |

| Network | Data transfer OUT (direct from EC2/ECS to internet, non-CDN) | $180 |

| Network | Inter-AZ data transfer (Multi-AZ RDS + ECS chatter) | $220 |

| Network | NAT Gateway data processing (~900GB/mo) | $405 |

| Network | NAT Gateway hourly (2 AZs × $0.045/hr × 720hr) | $65 |

| Network | VPC endpoints for S3/DynamoDB/etc. (~5 endpoints) | $50 |

| Monitoring | CloudWatch Logs ingestion (~120GB/mo) | $60 |

| Monitoring | CloudWatch Logs retention (~600GB) | $18 |

| Monitoring | CloudTrail data events | $22 |

| Monitoring | Datadog APM + Logs + Infra (external, ~$900) | — |

| Security | KMS key operations | $12 |

| Security | Secrets Manager (~40 secrets) | $16 |

| Security | GuardDuty | $95 |

| Security | AWS Config recorder | $35 |

| Misc | Route 53 hosted zones + queries | $12 |

| Misc | Support plan (Business tier, 3% of spend) | ~$130 |

| AWS subtotal | | ~$3,200–$3,400 |

| + external observability (Datadog) | | +$900 |

| Realistic total | | ~$4,100–$4,500 |

Where the Variance Comes From

• Data transfer is the largest source of month-to-month noise. A single popular customer with heavy media downloads can add $600 of CloudFront in a week.
• RDS storage growth is monotonic — it never goes down. Budget for +$40–$80/month in ongoing storage growth.
• NAT Gateway data processing scales with anything your private subnets talk to externally. Adding a new third-party API to your Lambda footprint can quietly add $80–$200/month.
• Datadog growth is proportional to feature-flag count and log verbosity. Most bills we audit have 30–50% of Datadog spend attributable to unused metrics and overly verbose debug logs.

The Cost-Optimisation Ladder (in order of ROI)

1. 1-year Compute Savings Plan on your steady-state ECS/EC2 footprint — 20–30% off compute for a one-line commitment.

2. VPC endpoints for S3, DynamoDB, and any AWS service your app talks to a lot. Cuts NAT gateway data processing dramatically. Break-even is typically <2 months.

3. CloudFront cache-hit ratio audit. If cache-hit is under 85%, tune cache-control headers on your API — every percentage point below 85% is meaningful money.

4. Move cold object storage to R2 or B2. If more than 30% of your S3 is >90 days old, the migration pays for itself in 3–4 months.

5. Log sampling in Datadog. Cap debug-level logs to 10% sample rate in production; the debugging value at 100% sampling is near-zero and the cost is real.

What Doesn't Move the Needle

• Ditching AWS for a "cheaper" cloud. The compute pricing gap doesn't justify the operational complexity for most SaaS at this scale.
• Aggressive right-sizing of a single service that's already under 5% of the bill.
• Reserved Instances beyond a 1-year Savings Plan. 3-year RIs lock you in for savings you can get from a much more flexible Savings Plan.

The Takeaway

A well-architected 100K MAU B2B SaaS should be spending $4,000–$5,000/month on AWS in 2026, with total infrastructure including third-party observability landing between $4,500 and $9,500 depending on data-transfer patterns. If you're materially over this, the culprit is almost certainly one of three things: unoptimised data transfer, un-cached CDN traffic, or an over-provisioned RDS instance from an earlier growth spurt that never got trimmed.