When fixed fee fits
A fixed fee works when deliverables, assumptions and acceptance criteria can be defined. Agree the boundaries and change process so both parties understand what the fee covers.
When time and materials fits
Time and materials can suit uncertain data quality, evolving scope or work dependent on other teams. Agree rates, an estimate, reporting and any spending cap before starting.
Review the unknowns first
For analysis work, a sample data review can reveal gaps that affect effort. For project management, meeting load, go-live windows and vendor dependencies affect the capacity required.
Ask for a visible change process
Whichever model you choose, understand how new requests are reviewed and approved. A pricing model is most useful when the underlying work stays visible.