Beyond unit price
The commercial number is only one part of a procurement decision.
Follow the operating effects
Minimum order quantities, delivery frequency and handling requirements can change the cost of using an item. Work with operations to describe those effects before comparing offers. The purpose is not to make a more elaborate spreadsheet; it is to avoid hiding important consequences.
Use consistent assumptions
Alternatives cannot be compared fairly if each uses a different demand pattern or service requirement. Agree the assumptions and identify where an estimate is uncertain. A transparent range is more useful than an apparently precise number without a clear basis.
Keep the trade-off visible
The preferred option may balance resilience, service and commercial cost rather than maximise one measure. Record the reasoning and the approval responsibility. That explanation becomes valuable when conditions change and the decision needs to be revisited.