A documented economic feasibility study,

this is, in a nutshell what a program business case is. The business case is used to establish validity of benefits to be delivered by a program.

The business case:

Is developed to assess the program’s investment against the intended benefits.
Links the organization’s strategic plan and the program management,
it’s done to help the organization to achieve these goals.
it is the primary justification document for an investment decision.
Defines the success criteria that is maintained through the program.
Enables to calculate the variance between the planned outcomes and the achieved outcomes.
It can be basic, high-level or detailed.