This content originally appeared on DEV Community and was authored by dejanualex
TL;DR Convince your client, organization to migrate to spot instances.
Assuming that you don’t have access to tools like AWS Cost Explorer, AWS Cost and Usage Reports, or AWS Cloud Intelligence Dashboard to examine your workloads and to retrieve estimated cost and usage information.
Don’t forget costs are driven by 3 factors: compute, storage, and networking (ingress is often free or at very low cost). You’re going to focus on the compute side of things as being a cost driver.
Step 1 Ask for a bill or estimated bill summary.
Step 2 Identify the pricing plan (Free Tier, On-Demand, Reserved Instances, or Savings Plans). Rule of thumb use on-demand (pay-as-you-go) for flexibility, reserved instances for predictable workloads, and free tier for eligible services.
Step 3 Do a cost estimation using AWS Pricing calculator to better understand the bill. AWS Pricing Calculator does not include any taxes and the prices for AWS services vary between Regions.
Step 4 Examine the compute usage and identify suitable services and workloads. Services like EKS, OpenSearch, CloudWatch, Kinesis, and Firehose suggest stateless/fault-tolerant/bath-oriented workloads suitable for Spot Instances. Therefore EKS worker nodes, data processing jobs, CI/CD workloads or OpenSearch indexing tasks can be migrated to Spot.
Step 5 Why Spot Instances? Spot Instances enable access to unused EC2 capacity at discounts of up to 90% compared to On-Demand pricing. To improve application availability on Spot be as diverse as possible in your instance type and Availability Zone selection.
Show potential savings i.e. average interruption frequency and savings over on-demand rates over last 30 days for m6i.xlarge instances.
Important note, prepare for Spot Instance Interruption. When Amazon EC2 interrupts (reclaims) a Spot Instance by default it will terminate the instance, unless you specify a different interruption behaviour, such as stop or hibernate.
Step 6 End you business case with confidence:
“Preliminary analysis indicates that the solution could benefit from Spot Instances as a substantial opportunity for cost optimization…The absence of Spot usage reflects untapped savings potential”.
Related read 
The art of guesstimating for estimation tips.
This content originally appeared on DEV Community and was authored by dejanualex