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Part 6 of 7 · Stock transfer planner series ~5 min read

What the stock transfer planner costs

There is no model in this system and the volume is the nightly imbalance scan: five thousand products across sixteen sites is eighty thousand pairs. Here is where each cent goes.

Key takeaways

  • About $122 a month at 80,000 pairs. Roughly $454 at 300,000 pairs.
  • One Bedrock read per SKU-site pair is the only line that scales. Everything else is rounding.
  • Nothing is always-on, so a quiet month genuinely costs almost nothing.
  • The nightly scan is the whole variable cost, and it is compute rather than storage.
  • The duplicate test runs before the read, so resends are free.
  • The three real risks: a retry loop, storage nobody expires, and a bigger model than the job needs.

The bill at three volumes

These are US East prices at the time of writing, at three volumes that bracket most small businesses. Find the bar closest to your own and read across.

Monthly cost of the stock transfer planner at three volumesA stacked bar chart with three bars, one per volume tier: 20,000 pairs totalling about $31, 80,000 pairs totalling about $122, and 300,000 pairs totalling about $454. Each bar is stacked from bands. The largest and fastest-growing is Bedrock, one read per SKU-site pair, in teal. Then SES for the messages, in pink. Then S3 and DynamoDB storage in green. Then a fixed orange band for Secrets Manager and AWS Budgets, which is eighty-six cents at every volume. Then a grey band for Lambda, SQS and CloudWatch. A note says the read is the only bar that grows with the business and the orange band never moves.$0$200$400$600$800~$31.4420,000 pairs~$122.0480,000 pairs~$454.24300,000 pairsBedrock — one read per SKU-site pairSES — asks, results, receiptsS3 + DynamoDBFixed — Secrets Manager, BudgetsLambda, SQS, CloudWatchThe read is the only bar that grows with the business. The orange fixed band never moves.
Fig 1. The monthly bill at three volumes. The teal band — one model read per SKU-site pair — is the only part that grows; the orange fixed band is the same 86 cents at every volume.

Line by line

LineAt 80,000 pairsHow it scales
Bedrock read$0.00Linear. One call per SKU-site pair, roughly 1,800 in and 200 out tokens.
SES$40.00Linear. About 0 messages per SKU-site pair.
DynamoDB + S3$13.08Storage grows with what you retain, not with throughput.
Lambda + SQS$0.12Linear, and effectively free at this scale.
CloudWatch$0.16Flat, if you set retention. Unbounded if you do not.
Secrets Manager$0.40Flat. One secret, $0.40 a month.
AWS Budgets$0.46Flat. Two actions, so you find out before the bill does.

There is no read line and no messaging line: the output is a pick list, not an email. Compute for the nightly scan is the entire variable cost.

The three ways this bill surprises you

Every one of these has happened to somebody, and all three are cheap to prevent.

  • Scanning every pair every night. Only products whose stock or sales changed need re-evaluating, which is a small fraction on any given day.
  • Recomputing demand history per pair per night. Weekly demand rates move slowly. Compute them once a week and cache.
  • Storing a proposal per pair. Only proposals that clear the economic test are worth writing; the rejected ones are a count, not a record.

What it costs when nothing happens

This matters more than the headline number for a seasonal business. In a month with nothing to process the bill is the fixed band: Secrets Manager at forty cents, AWS Budgets at forty-six, and a few cents of storage. Call it a dollar. There is no instance to stop and nothing to remember to turn off.

The monthly bill at four volumes plus one failure modeA horizontal row of five boxes. Quiet month, about one dollar. 20,000 pairs, about $31. 80,000 pairs, about $122. 300,000 pairs, about $454. And one bad retry loop, about two hundred dollars. A note says four of these are the design working and the fifth is a missing dead-letter queue.THE BILL, AT A GLANCEQuiet month~$120,000 pairs~$3180,000 pairs~$122300,000 pairs~$454One bad loop~$200Four of these are the design working. The fifth is a missing dead-letter queue.
Fig 2. The bill at a glance, including the one that is not a volume at all. A retry loop with no dead-letter queue costs more than every legitimate use of the system put together.
  • Management
  • Analytics
  • Front-end & mobile

Set these on day one

  • A dead-letter queue on every SQS queue, with a maximum receive count of three.
  • Thirty-day retention on every CloudWatch log group. There is no default that is safe.
  • An S3 lifecycle rule on the object prefix, tiering at 90 days and expiring at your actual record-keeping horizon.
  • Two AWS Budgets actions — one that emails at half your expected spend, one at double it. The second is how you find out about a loop in an hour instead of a month.
  • Provisioned concurrency: none. Nothing here is latency-sensitive enough to justify paying for a warm function.

Next: the same system drawn for engineers — service names, resource identifiers, IAM scopes, table schemas and the model id.

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