Serverless Cost: When It Wins, When It Doesn’t

Serverless economics are uniquely shaped. The threshold is well-defined but rarely calculated.

Serverless cost shape

Serverless economics have a specific shape: high per-request cost, zero idle cost. The crossover with provisioned compute is at a measurable request rate.

• Per-request cost. High compared to provisioned compute; the bill scales with traffic linearly.
• Scaled-to-zero. No idle cost; the killer feature for sporadic workloads.
• Crossover. Per-service crossover with provisioned at a specific request rate; below it, serverless is unbeatable.
• Above crossover. Provisioned wins; sustained traffic does not benefit from scale-to-zero.

Four thresholds

• Lambda crossover: ~10M requests/month for typical compute.
• Fargate crossover: ~24/7 utilization at 70%+.
• API Gateway crossover: ~50M req/month vs ALB.
• DynamoDB on-demand crossover: ~50K writes/day.

Migration patterns

When serverless stops paying, migration to provisioned is the answer. The mechanics depend on whether the workload is stateless or stateful.

• Stateless migration. Lambda to ECS/EKS/EC2 is mechanical; same code, different runtime.
• Stateful migration. DynamoDB on-demand to provisioned: capacity planning becomes the new problem.
• Connection model. Serverless connections are short-lived; provisioned can hold pools; the app may need adjustment.
• Cutover plan. Run both in parallel during cutover; flip traffic gradually; rollback is one DNS change.

Hybrid posture

Most mature orgs run a hybrid. Serverless for the sporadic, provisioned for the sustained; the realistic posture, not a religious one.

• Serverless for sporadic. Cron jobs, infrequent APIs, async background work; scale-to-zero pays.
• Provisioned for sustained. High-volume APIs, long-running workers; provisioned cost dominates serverless premium.
• Per-service decision. Each service picks based on traffic profile; do not force one runtime org-wide.
• Threshold tracking. Quarterly review of crossover; services migrate as their traffic profile shifts.

Antipatterns

• Serverless forever despite high volume. 3-5x bill premium.
• Provisioned for sporadic. Pay for idle.
• No threshold tracking. Crossover passes silently.

What to do this week

Three moves. (1) Apply this lever to your highest-spend workload. (2) Measure the dollar impact for one month. (3) Roll the practice out to the next two services if the savings hold.