Your first definition is correct: Planned Benefit = Revenue Generated by the Investment
Costs go into the Cost Plan. Financial metrics in CA PPM use Revenue from Benefit Plan and Costs in Cost Plan to return values. If one leaves Cost Plan empty, putting net result in Benefit Plan, you'll either get 0 or infinite result depending on the calculation. Also, if one just puts costs in Cost Plan and nothing in Benefit plan, one will get the reverse - infinite or 0 results depending on the calculation.
What's more is that putting Revenue in the Cost Plan is a futile exercise - the financial metrics ignore Revenue placed in the Cost Plan. Of course, workarounds are possible, where some can be potentially ugly (e.g. scheduled job or job available from Action menu to calculate Revenue in Cost Plan (rate x units) and copy to Benefit Plan -- now, revenue is visible/editable in two locations).
See idea and vote, please: Financial Calculations to include Revenue from Cost Plan -- This idea has been 'under review' since 2012, but only in the current Idea system -- it's true 'create date' goes back to Niku 6 days, something like 2002.