Including the Expense Totals will create the row on the report with "Expensed Total", "Invoiced Total", and the difference between those two, "Net". The purpose of that row is to give the User an idea of the outcome of the linked items -- was it worth the money we spent to close the customers that we did.
Expensed Total is the sum of all the linked expense amounts (doesn't include the optional "tax" field on the expenses).
Invoiced Total is the sum of the sales amounts on Invoices for Prospects that were closed with a "Won". That is linked Opportunities that had a Close Reason of a "Business Won" selection.
Net is obtained from subtracting the expenses from the invoiced amounts. This is the total (hopefully positive) outcome of this project.
If you would like to include the cost of salary/time as part of the calculation, add an expense report that shows that estimate and link it to the project.
Including the Opportunity Totals on the report creates the second line, under the expense items. That is "Total Prospects", "Won Prospects", and "Close Rate". These entries show a summary of the deals that were worked on and linked to this project.
Total Prospects comes from the linked Opportunities, and is just a total of those.
Won Prospects is a total of the Opportunities that had a Close Reason of the "Business Won" type.
Close Rate is obtained by dividing the Won Prospects by the Total Prospects.
Have Opportunities that didn't directly occur at the event in question, but were a result of referrals or follow-ups? Opportunities can be linked to multiple Projects, so it is possible to "credit" it to multiple entries for tracking purposes. Just be careful not to double-count the numbers when adding them up.