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Part 6 of 7 · Event RSVP manager series ~3 min read

What the event RSVP manager costs

This is one of the cheapest systems in the whole series, and almost all of the bill is the emails it sends. A confirmation per guest, a couple of reminders each, the odd waitlist offer — that’s the cost. The Lambda runs, the database writes, and the timed jobs are all pennies. Bedrock only fires twice in the life of an event: the post-event thank-you note and a small host Q&A helper. For a typical 200-guest event, the whole thing lands around $2.40.

Key takeaways

  • Around $2.40 for a typical event of about 200 guests.
  • Fixed AWS cost is essentially zero. No always-on compute, no NAT Gateway, no API Gateway.
  • Most of the bill is SES email — $0.10 per thousand messages sent.
  • Bedrock fires only on the thank-you note and the host Q&A helper — cents per event.
  • A 50-guest event lands near $0.80. A 1,000-guest conference lands around $9.

Cost at three event sizes

Cost per event at three guest counts, broken out by component A horizontal stacked-bar chart showing the cost per event in US dollars at three guest counts. The leftmost bar represents a 50-guest event and shows a total around $0.80, dominated by the SES email slice (confirmations, reminders, and waitlist offers) with a tiny sliver for Lambda, DynamoDB, and Scheduler, a tiny Bedrock sliver, and a tiny fixed sliver. The middle bar represents a 200-guest event and shows a total around $2.40, with the same shape — the email slice grows roughly linearly with guest count because each guest gets a confirmation plus a couple of reminders. The rightmost bar represents a 1,000-guest conference and shows a total around $9, with email still dominant; Bedrock and the fixed amounts stay small in absolute terms because they only fire on the post-event thank-you note and the host Q&A helper. Below the chart is a legend explaining the four sections of each bar: SES email (confirmations, reminders, waitlist offers), Bedrock (thank-you note and host Q&A helper only), AWS Budgets and Secrets Manager (small fixed amounts), and an everything-else bucket for Lambda runtime, DynamoDB on-demand, S3, and EventBridge Scheduler. A note at the bottom: the emails are the dominant cost, and even those are a tenth of a cent each. $0 $5 $10 $15 $20 50 guests ~$0.80 200 guests ~$2.40 1,000 guests ~$9 SES email (confirmations, reminders, offers) Bedrock (thank-you note + host Q&A helper) AWS Budgets + Secrets Manager (fixed) Everything else (Lambda, DDB, S3, Scheduler) The emails are the dominant cost — and even those are a tenth of a cent each.
Fig 6. Cost per event at three guest counts. SES email is the dominant slice because every guest gets a confirmation plus a couple of reminders. Bedrock and the fixed amounts stay tiny because they fire at most twice per event.

Where the dollars actually go

SES email (the bulk). Every guest gets one confirmation and, on the default schedule, up to three reminders. Add waitlist offers and the odd cancellation acknowledgment, and a 200-guest event sends on the order of 700 to 900 emails. At SES’s $0.10 per thousand, that’s under ten cents in raw send fees — but the email slice on the chart also folds in the small per-message Lambda and template-rendering work, which is why it’s the dominant band. Either way, the whole email lane is a couple of dollars at most at this size.

Lambda runtime. A register click, a confirm, a cancel, a claim, each reminder fire — every one is a short Lambda invocation measured in a few hundred milliseconds. Even a 1,000-guest event is a few thousand invocations spread over weeks. Pennies a month at every size.

DynamoDB on-demand. Three small tables hold the guest list, the live count, and the audit trail. The conditional write that defends the cap is a single update per seat change. Reads are the host headcount and the duplicate checks. Pennies at any of these volumes.

EventBridge Scheduler. Each confirmed guest books up to three reminder jobs, plus a waitlist timeout per offer. A 200-guest event books a few hundred one-off jobs over its life. Scheduler is fractions of a cent per job. Negligible.

S3 + storage. The inbound email lane writes raw replies to S3; the register page assets and a small settings record round it out. A few hundred KB total. Effectively free.

Bedrock (only twice). The system uses no model on any hot path — not on register, confirm, cancel, claim, or reminders. Bedrock Haiku 4.5 fires only on the post-event thank-you note (one call that writes a warm wrap-up the host can edit and send to the confirmed list) and the host Q&A helper (a small assistant that answers the host’s plain-English questions about their own guest list, like “how many seats are still open?”). A couple of cents per event.

What doesn’t cost money

  • API Gateway. Replaced by Lambda Function URLs for register, confirm, cancel, and claim.
  • NAT Gateway. Nothing is in a VPC. No NAT, no $32/month minimum.
  • Always-on compute. No EC2, no Fargate, no reminder poller. Each reminder is its own alarm; the system sleeps in between.
  • A database server. DynamoDB on-demand means no provisioned capacity to pay for between events.
  • Models on the hot path. The cap, the waitlist, and the reminders are plain code. Bedrock fires only on the thank-you note and the host Q&A helper.

How the cost scales

The bill scales with guest count, because more guests means more emails, more conditional writes, and more reminder jobs — all roughly linear. Bedrock doesn’t scale with guests at all; it’s a flat couple of cents whether the event has 50 people or 1,000. So a 2,000-guest event lands around $18, and a busy host running four 200-guest events a month spends under $10 across all of them. Past a few thousand guests per event you’d batch the reminder sends rather than book one job per guest, but that’s an optimization for large conferences, not a redesign.

Set an AWS Budgets alarm at $15/month so anything unusual pages you before the bill matters. A normal small-event host stays comfortably under that.

Last post in the series: the engineering reference. Same system, drawn for engineers — service names, Lambda inventory, IAM scopes, the conditional-write capacity logic, DynamoDB schemas, and the EventBridge Scheduler config.

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