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Part 6 of 7 · Subscription audit bot series ~5 min read

What the subscription audit bot costs

This is the cheapest system in the series, and the reason is structural: the model is only ever asked about a merchant string it has not seen before. After the first pass resolves a business’s forty-odd merchants, the ongoing cost is a handful of new ones a month. Here is where each cent goes.

Key takeaways

  • About $2 a month at 120 new merchants. Roughly $3 at 400 new merchants.
  • One Bedrock read per merchant is the only line that scales. Everything else is rounding.
  • Nothing is always-on, so a quiet month genuinely costs almost nothing.
  • The model only runs on merchant strings never seen before, so the second month costs a fraction of the first.
  • 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 subscription audit bot at three volumesA stacked bar chart with three bars, one per volume tier: 40 new merchants totalling about $1, 120 new merchants totalling about $2, and 400 new merchants totalling about $3. Each bar is stacked from bands. The largest and fastest-growing is Bedrock, one read per merchant, 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$1$2$3$4~$1.3840 new merchants~$1.68120 new merchants~$2.73400 new merchantsBedrock — one read per merchantSES — 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 merchant — is the only part that grows; the orange fixed band is the same 86 cents at every volume.

Line by line

LineAt 120 new merchantsHow it scales
Bedrock read$0.25Linear. One call per merchant, roughly 1,800 in and 200 out tokens.
SES$0.07Linear. About 1.2 messages per merchant.
DynamoDB + S3$0.30Storage 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.

The volume unit here is deliberately unusual: it is new merchant strings, not transactions. Transactions are free to process; only an unrecognised merchant costs a model call, and a settled business produces very few of those.

The three ways this bill surprises you

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

  • Re-resolving known merchants. If the known-merchants cache is keyed on the raw descriptor rather than the cleaned one, every trivial variation of the same string is a fresh model call. Key it on the cleaned merchant and the first pass is the only expensive one.
  • Re-ingesting overlapping exports. Without the line fingerprint, re-uploading a statement doubles charges, which breaks grouping and can produce a flurry of false new-subscription alerts.
  • Log retention left at never. With a few hundred transactions a month the logs will out-cost everything else within a year. Thirty days of retention is the whole fix.

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. 40 new merchants, about $1. 120 new merchants, about $2. 400 new merchants, about $3. 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~$140 new merchants~$1120 new merchants~$2400 new merchants~$3One 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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