Be careful how you measure things, sometimes there will actually be savings here, often there will be costs that would have been incurred regardless of Wikimania. For example two chapter staff members at Wikimania discuss something in person that they would have discussed by phone. If you count the cost of their time at Wikimania then you might cost it as 3 days of each of their time for Wikimania. But if a staff member is using part of their time at Wikimania to meet people who they would otherwise have called then it would be wrong to count their full salary cost for those three days against the Wikimania budget. There may even be a saving on international phone calls.
This could be quite significant as there were many WMF and Chapter staff at Wikimania. Some of their time will be because of Wikimania, but I suspect that much, hopefully most of their time will have been spent on things that Wikimania enabled. So a panel of developers taking questions from the community at Wikimania should in my view be counted as part of the community liaison work of those developers. To some extent this is an accounting exercise. There is a natural desire to get a "true cost" of Wikimania. But it would be easy to get a false picture of that cost if you simply count the cost of employing all those who are staff and also the travel and accommodation.
Regards