How to figure out cost per tenant in multi-tenant environment

You can use AWS Cost Categories to map your AWS costs and usage into meaningful categories. With cost categories, you can organize your costs using a rule-based engine. The rules that you configure organize your costs into categories. For shared costs, you can use Split Charge Rules, a feature of Cost categories to split the costs using fixed percentage, equal or proportionately. https://docs.aws.amazon.com/awsaccountbilling/latest/aboutv2/splitcharge-cost-categories.html

For usage based cost allocations, you can use Split Cost Allocation Data, a feature of Cost Management that allows you to split the charge compute costs of ECS and Batch workloads. https://docs.aws.amazon.com/cur/latest/userguide/split-cost-allocation-data.html

The AWS Cost Profiler is designed for this:

• Introducing AWS Application Cost Profiler
• Application Cost Profiler workshop
• CostLenz: Track AWS costs per tenant and application by extracting usage data for AWS Application Cost Profiler from OpenTelemetry traces (This is a 3rd party service and cost money)

I'm afraid there is no silver bullet for this, but rather ask how much of your total cost is currently unallocated to one of the customers? I wouldn't aim for 100% here because taking care of last 10-20% is likely to more costly than what it is worth. Ideally you could have dedicated accounts for every customer but that wouldn't work with multi-tenant thinking and would increase total costs compared to your current model where you can share some resources. Typically you would allocate those shared costs a) by equal split for each customer, b) proportinally to their allocated spend or some other business metric you have. You could also check https://www.finops.org/framework/capabilities/manage-shared-cloud-cost/ for inspiration what to do with shared cost.