Programs are undertaken for many reasons.
Some programs seek to solve a problem some others address a specific organizational need.
Generally speaking, program benefits are difficult to evaluate, especially for those programs that are opportunity oriented.
The suggestion is to begin the benefit evaluation by considering the stakeholders who are expected to benefit from the results of the program.
The longer the program, the more uncertain the estimated benefits will be.
It is important to assemble the best estimates of expected returns. For marketable products/services set out the revenue streams expected.
Keep in mind: be skeptical of assessments that appear too optimistic. It would be worthwhile to review the assessment periodically after program has started.