How can I find out my cost per user onboarding?

Hi, You might consider to add tag to see the cost per user. https://docs.aws.amazon.com/awsaccountbilling/latest/aboutv2/cost-alloc-tags.html

Thanks.

Tagging could be one option, however I feel like you might need more than that. It really depends which AWS services you are going to use. Tagging can only help to an extent where you'd need to group multiple AWS resources (e.g. tag several EC2 instances, RDS databases, ELB load balancers, or S3 buckets with the Tag key "application" and Tag value "%name%"); therefore, you'd be able to use them for example in Cost Explorer to view cost by application.

However, I assume that you might have multiple users signing up / using your application, in which case you likely need to allocate this cost of your workload to (let's say) several hundreds sign ups or users. There's no easy AWS-native tool or solution that would do this out of the box, and you would need to combine multiple metrics to get this information. Tagging would help to an extent, but you effectively need to know resource utilization metrics (e.g. you could use CloudWatch for example, to get more granular resource metrics). As for the user data, because AWS doesn't look inside your application, this is something you'd have to check on your side... metrics like number of sign ups, number of users, etc... these would be something you'd have to monitor on your side.