How to use this tool

Infrastructure Cost Modeller — Guide

How to read the cost breakdown, what each AWS service contributes per transaction, and how to use the output for budget planning and architecture decisions.

What is this tool for?

When you deploy an agent-based payment system on AWS, your infrastructure cost is not a single number — it is a function of transaction volume, the services you enable, and the tier of scaling you have reached. This tool models that cost per transaction and per month across four growth stages.

Each cost figure is derived from published AWS pricing for eu-west-1 (Ireland) on-demand rates, converted to GBP. The model breaks down Lambda, DynamoDB, SQS, KMS, Secrets Manager, X-Ray, CloudWatch, NAT Gateway, ECS Fargate, Bedrock, Aurora, ElastiCache, WAF, and API Gateway — every service in a standard agent payment stack.

Step-by-step walkthrough

Step 1 — Select your growth tier
  1. Click one of the four tier cards at the top: Early, Growth, Scale, or Enterprise.
  2. Each tier activates a different set of AWS services — for example, ECS Fargate and ElastiCache only appear from the Growth tier onwards.
  3. Pick the tier that most closely matches your current architecture, not your aspirational target.
Step 2 — Set your monthly transaction volume
  1. Use the slider to set your estimated monthly transaction count. The range adjusts to match the selected tier.
  2. The cost per transaction and estimated monthly total update in real time as you drag.
  3. The per-tx cost is the most useful figure for unit economics — compare it against your payment processing margin.
Step 3 — Toggle optional services
  1. Bedrock (ML fraud): toggle off if you are not yet using AI-based fraud scoring.
  2. Aurora RDS: toggle on if you have a relational settlement ledger in addition to DynamoDB.
  3. ElastiCache: toggle on if you use Redis for idempotency key storage.
  4. WAF: toggle on if you have WAFv2 applied to your API Gateway or CloudFront distribution.
  5. Each toggle updates the monthly cost and per-tx cost immediately.
Step 4 — Read the cost breakdown
  1. The breakdown panel shows each service's contribution to the per-tx cost and fixed monthly cost.
  2. Services with a fixed monthly cost (ECS, Aurora, ElastiCache) are shown separately from per-tx services (Lambda, DynamoDB, SQS).
  3. Use the Export button to download the breakdown as a CSV for budget spreadsheets or board decks.

Growth tiers explained

Early — 0–100K tx/mo
Pre-PMF. Lambda + DynamoDB on-demand. Minimal fixed costs. Bedrock Haiku fraud checks optional.
Growth — 100K–1M tx/mo
Series A/B. ECS Fargate settlement agent added. ElastiCache idempotency layer. Full Bedrock fraud ML.
Scale — 1M–10M tx/mo
RDS Aurora for settlement ledger. Multi-AZ. Reserved capacity on Lambda and DynamoDB to reduce per-tx cost.
Enterprise — 10M+ tx/mo
Multi-region active-active. Provisioned DynamoDB. CloudHSM for key management. Full enterprise AWS support.

Per-service cost reference

All figures are AWS eu-west-1 on-demand pricing converted to GBP (~0.79 USD→GBP). Assumes average 512MB Lambda, 500ms execution, 3 DynamoDB writes + 5 reads per transaction, 4 SQS messages per transaction.
LambdaRequest + GB-s~£0.000118 / tx
DynamoDBOn-demand reads + writes~£0.000004 / tx
SQS4 messages / tx~£0.0000003 / tx
Secrets Manager1 API call / tx (cached)~£0.0000039 / tx
KMS2 API calls / tx~£0.0000047 / tx
X-Ray5% sampling~£0.0000002 / tx
CloudWatch Metrics3 custom metrics / tx~£0.0000237 / tx
NAT Gateway2KB data / tx~£0.0000001 / tx
API Gateway HTTP$1.00/M requests~£0.00000079 / tx
ECS Fargate2 tasks, fixed monthly~£35/mo base
Bedrock Haiku500 in + 100 out tokens / tx~£0.0000216 / tx
Aurora Serverless v2Min 2 ACU, fixed monthly~£136/mo base
ElastiCache r7g.largeFixed monthly~£94/mo base
WAF$5/mo + $1/M requests~£4/mo + per-tx
Estimates only. AWS pricing changes periodically. Reserved instances, Savings Plans, and volume discounts can reduce costs by 30–60% at scale. Verify costs in the AWS Pricing Calculator before finalising budget forecasts.

Key terms

On-demand pricingAWS billing model where you pay per request or per GB-s of usage. No upfront commitment. Higher per-unit cost than reserved, but no risk of over-provisioning.
Per-tx costInfrastructure cost per individual payment transaction. Used for unit economics: if your AWS cost per transaction is £0.0002 and your payment margin is £0.02, your infrastructure overhead is 1%.
Fixed monthly costCosts that do not scale with transaction volume — ECS Fargate tasks, Aurora instances, ElastiCache nodes. These must be paid regardless of traffic.
Provisioned capacityDynamoDB billing mode where you pre-specify read and write capacity units. Cheaper than on-demand at consistent, predictable load. Used at Scale and Enterprise tiers.
Bedrock HaikuAWS Bedrock model: Claude Haiku. The lowest-latency, lowest-cost Claude model. Used for fraud scoring in the cost model — approximately 3–5× cheaper than Claude Sonnet per token.
ACUAurora Capacity Unit — the scaling unit for Aurora Serverless v2. Each ACU provides approximately 2GB of RAM. Billing is per second of ACU usage.

Frequently asked questions

Ready to model your infrastructure costs?
Select your growth tier, set your transaction volume, and get a per-tx cost breakdown in seconds.