[AI/BI] Build costing model

## Summary
Build a costing model for AI/BI migration / service engagements.

## Notes
WIP — already in progress.

## Acceptance criteria
- [ ] Costing model draft available
- [ ] Inputs/assumptions documented
- [ ] Reviewed with track stakeholders
- [ ] Linked from track Task Tracker

## Track
`ai-bi-migration`

## Status
In Progress