The costs will be largely extrapolated from the work done in the schedule. Complete the calculations in the spreadsheet and identify any purchases. The spreadsheet is only for 6 months so you may need to extend the columns if the project goes for longer.
The benefit side of the equation will involve getting commitment from people as to what benefits the project will deliver. The information gathered will go into a Preliminary Business Case. At a later point the business case can be refined and benefits reviewed. Unless people can commit to benefits they should not be included in the project. The Benefit User Guide provides much more detailed information on benefit identification and quantification.
Once costs and benefits are completed, you can complete the profitability analysis which shows how long to break even, ROI etc.