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Part 6 of 7 · Shelf price checker series ~5 min read

What the shelf price checker costs

The model reads photographs, and a photograph costs more to read than a line of text. Everything else here — the price file, the promotions calendar, the matching and the ranking — is arithmetic over data the till system already exports. Three thousand photos a month is roughly the five-store chain in this series: a full walk of every store once a month and the changed bays every week. Here is where each cent goes.

Key takeaways

  • About $25 a month at 3,000 photos. Roughly $79 at 10,000 photos.
  • One Bedrock read per photo is the only line that scales. Everything else is rounding.
  • Nothing is always-on, so a quiet month genuinely costs almost nothing.
  • The bill tracks photos, not labels or stores. Walking only the changed bays each week is what keeps it at this size.
  • 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 shelf price checker at three volumesA stacked bar chart with three bars, one per volume tier: 800 photos totalling about $8, 3,000 photos totalling about $25, and 10,000 photos totalling about $79. Each bar is stacked from bands. The largest and fastest-growing is Bedrock, one read per photo, 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$25$50$75$100~$7.6800 photos~$24.763,000 photos~$79.3610,000 photosBedrock — one read per photoSES — 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 photo — is the only part that grows; the orange fixed band is the same 86 cents at every volume.

Line by line

LineAt 3,000 photosHow it scales
Bedrock read$18.00Linear. One call per photo, roughly 1,800 in and 200 out tokens.
SES$1.65Linear. About 0.5 messages per photo.
DynamoDB + S3$1.60Storage 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.

Roughly one model call per shelf photo, against a small vision-capable model. A photo of a shelf section holds about seven labels, and the read returns every one of them as printed characters with a position in the frame. That is transcription from an image rather than judgement, and a frontier model reads the same seven labels for several times the price.

The three ways this bill surprises you

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

  • Walking every bay every week. It more than doubles the read line and finds very little the changed-bay walk does not. The full walk is for drift, and drift is monthly.
  • Sending the price file to the model. It is tens of thousands of fixed-format rows and the largest input the system touches. Parse it. It is also the one thing the read must never see.
  • Keeping every photo forever. A photo is evidence until its exceptions close. After that the label record is the evidence, and the image, which may have caught a customer in the aisle, should expire.

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. 800 photos, about $8. 3,000 photos, about $25. 10,000 photos, about $79. 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~$1800 photos~$83,000 photos~$2510,000 photos~$79One 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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