# Cost Categories split-charge rules for shared cost (2026)

Companion artifact for the FactualMinds post **AWS Tagging, Chargeback, and FinOps Ownership**.
Use **AWS Cost Categories** to allocate the ~15–20% of the bill that no tag can ever reach (egress,
inter-AZ transfer, Enterprise Support, shared platform services). This replaces the hand-built
allocation spreadsheet that finance and engineering argue over every month.

## Concepts

A split charge rule has three parts:
- **Source** — the shared cost pool to distribute (e.g. the `Shared-Platform` cost category value).
- **Targets** — the cost category values that receive the allocation (e.g. each `Team` value).
- **Allocation method** — `PROPORTIONAL` (by each target's own spend), `FIXED` (set percentages),
  or `EVEN` (equal split).

Rules are evaluated **top-down**, so the shared cost category values must sit **at the top** of the
rule list, before the rules that consume them.

## Recommended cost category: `Team`

1. Define a `Team` cost category with rules mapping `CostCenter` tag → team value.
2. Add a `Shared-Platform` value that captures untaggable line items by charge type / service
   (data transfer, support, KMS/Config/CloudTrail, EKS control plane).
3. Add a split charge rule:

```
Source:  Team = Shared-Platform
Targets: Team = platform, payments, data, growth, ml
Method:  PROPORTIONAL   # each team absorbs shared cost in proportion to its own direct spend
```

## Which method for which shared cost

| Shared cost | Method | Why |
|-------------|--------|-----|
| Egress + inter-AZ transfer | `PROPORTIONAL` | Heavier-compute teams generally move more data |
| Enterprise Support | `EVEN` *(then revisit)* | Support is a flat fee; even-split avoids penalizing the biggest spender for a fixed cost. Switch to proportional only if leadership prefers it. |
| Platform security (KMS, Config, CloudTrail, GuardDuty) | `PROPORTIONAL` | Scales loosely with footprint |
| EKS control plane + shared add-ons | `PROPORTIONAL` | Pod-level cost handled separately by split cost allocation data |

## Order of operations (matches the post's "Monday" list)

1. Build the `Team` cost category from the `CostCenter` tag — visible in Cost Explorer / Budgets /
   CUR from the **start of the month** you create it (not retroactive).
2. Add the `Shared-Platform` value at the **top** of the rule list.
3. Add one `PROPORTIONAL` split rule first. Resist per-cost fixed percentages until a team disputes
   a number — only then introduce a `FIXED` rule for that specific line.
4. Publish the allocated view as **showback** for one cycle. Switch to chargeback only after the
   numbers stop moving.