An analysis of the cost-benefit trade-offs for a project should be included as part of the:

The work plan (project management plan's schedule component) focuses on activities and timelines, not cost-benefit trade-off justifications.

The risk register documents identified risks, their probability, and responses - it does not capture cost-benefit analyses for project trade-offs.

While the project charter may reference a high-level business case, it is an authorization document and does not contain the detailed cost-benefit trade-off analysis associated with quality planning.

In the PMBOK Guide, cost-benefit analysis is explicitly listed as a tool and technique for the Plan Quality Management process. The resulting Quality Management Plan documents the outcomes of these trade-off analyses, justifying quality activities by comparing the cost of conformance against the cost of non-conformance.