According to the Forecasts Administrator's Workbook - http://login.salesforce.com/help/pdfs/en/forecasts.pdf - it should be possible to forecast by expected revenue (Step 3 on pages 5 and 6).
However, in the Forecasts Settings, I can only select Opportunities Revenue, Opportunities Quantity, Product Families Revenue or Product Families Quantity.
When I google for this problem, I come across the Salesforce Ideas forum, where people request this feature (forecast by expected revenue), but it does not seem to be possible at this moment.
Something else that I tried. I quote from the Workbook: "Custom Opportunity Currency Field - Revenue = The rollup is based on the amount in the custom opportunity currency field that you specify". So I added a custom currency field to Opportunity, filled it for some records, went to Forecast Setup. Only to see that this type also is not available.
Next I enabled Opportunity splits, because https://help.salesforce.com/apex/HTViewHelpDoc?id=forecasts3_understanding_custom_field_forecasts.htm&language=en_US states: "To enable a custom field forecast, your administrator also has to enable opportunity splits for the custom field". That doesn't sound very logical and - surprise, surprise - it doesn't solve the problem. I can now enable additional Forecast types, those that have to do with splits, but I still cannot forecast by expected revenue, or by custom currency field.
Based on the same help page as quoted above and the fact that I can enable Opportunity split forecasting, I can conclude that the org that I am using has Collaborative forecasting.
Can anyone help me please?