Typically, most non-operating costs are due to either construction, or the paying down of revenue bonds from earlier construction. These costs are among those included in the airport's overall costs for the year, hence they are passed on to the tenants (who, of course, pass them on to the paying passenger).
As for construction, and this varies according to the type of poject, the FAA will pay from 75 to 90 percent of the project cost, with the airport picking up the rest (and these are also included in the budget).
Occasionally, for projects that will not directly affect the aeronautical use of the airport, or the traveling public in accessing the airport, the airport will have to pick up the entire cost. An example of this would be an on-airport hotel, such that MSY is about to embark upon.
Airport income and expense budgeting is not an excat science. There's a lot of planning involved, using past historical budgets and expense reports, crystal ball gazing into the future, etc. One thing to note is that while it's tough to exactly balance the airport budget, there are items in place to either increase revenues from year to year in case of shortfalls, or has happened before, give tenants credits for overages.
This is a very complex subject (MSY has 8-odd people in our Accounting department, that this stuf is all they do), and let me tell you, some of it is tough to understand.
Tom in NO (at MSY)
"The criminal ineptitude makes you furious"-Bruce Springsteen, after seeing firsthand the damage from Hurricane Katrina