If ROI is total benefits/total costs, you're going to have to get some valid data. Hence the need for friends in accounting.
The Return Calculations tab, partially shown below, lists only two forms of revenue: Direct (sales and other contracts tied directly to the proposal) and Indirect, which has varying degrees of significance, depending on the culture of the client's business.
If you need to add an itemised line for another form of revenue, just use Excel's Insert command to drop a whole new row in the range.

Again, it is strongly recommended that some information about current revenue streams is included to give the proposal more credibility. The spreadsheet will net out the values you enter for both scenarios and report those figures to the Overall ROI Calculation tab.
In our sample project, a fairly flat rate of revenue is assumed, given that insourcing an application is essentially a cost-cutting proposition.