Skip to content
allanninal.dev/build
Collaborate

Part 6 of 7 · Win-back campaigner series ~5 min read

What the win-back campaigner costs

There is no model in this system and the send volume is a fraction of the list, because suppression removes about a third before anything is sent. Six hundred lapsed customers a quarter is a busy small retailer. Here is where each cent goes.

Key takeaways

  • About $2 a month at 600 lapsed. Roughly $5 at 2,500 lapsed.
  • One Bedrock read per lapsed customer is the only line that scales. Everything else is rounding.
  • Nothing is always-on, so a quiet month genuinely costs almost nothing.
  • Two thirds of the list gets a message; the rest are suppressed and cost nothing to not contact.
  • 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 win-back campaigner at three volumesA stacked bar chart with three bars, one per volume tier: 150 lapsed totalling about $1, 600 lapsed totalling about $2, and 2,500 lapsed totalling about $5. Each bar is stacked from bands. The largest and fastest-growing is Bedrock, one read per lapsed customer, 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$2$4$6$8~$1.47150 lapsed~$2.19600 lapsed~$5.182,500 lapsedBedrock — one read per lapsed customerSES — 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 lapsed customer — is the only part that grows; the orange fixed band is the same 86 cents at every volume.

Line by line

LineAt 600 lapsedHow it scales
Bedrock read$0.00Linear. One call per lapsed customer, roughly 1,800 in and 200 out tokens.
SES$0.34Linear. About 0.65 messages per lapsed customer.
DynamoDB + S3$0.38Storage 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: nothing here calls a model. The messaging band assumes about two thirds of the lapsed list passes suppression.

The three ways this bill surprises you

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

  • Recomputing every customer’s interval nightly. Intervals move slowly. Once a week is plenty and cuts the compute by most of it.
  • Scanning the whole order table. The detector queries by customer and product against an index; a scan across a few years of orders is the one line item here that can get genuinely expensive.
  • Storing a full order history copy. This system reads the orders that already exist. A second copy is cost and a synchronisation problem.

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. 150 lapsed, about $1. 600 lapsed, about $2. 2,500 lapsed, about $5. 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~$1150 lapsed~$1600 lapsed~$22,500 lapsed~$5One 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.

All posts