My last post discussed the year-to-year budgeting of DebConf. This post discusses what makes budgeting difficult within one particular DebConf. Since every DebConf is unique in its arrangements, the particular difficulties always vary year-to-year. For example, DebConf10 was easy in that Software in the Public Interest was in the same country so that we had easy access to bank accounts, but more difficult venue payment logistics. I'll use DebConf10 as an example below.
For DebConf10, we get our venue "for free" thanks to the support of the Columbia Computer Science department. However, we had to pay for guard costs after building hours, and custodial costs. For custodial costs at least, we had to go through many layers of bureaucracy, and people kept insisting it would be more than they quoted. (It ended up being about half, since DebConf attendees were so responsible!)
DebConf sponsors many attendees. This means that all their food and accommodation costs are paid by us. We have sponsored registration and reconfirmation deadlines to set a maximum of the number of people we have to sponsor, but people are continually dropping out and it's hard to know just how many people to reserve for. Sponsored people won't always eat every meal with us, and usually won't decide until the day before - so figuring out food amounts is hard, too. Furthermore, we have unsponsored people who will want food and accommodations (paying themselves) which we must somehow plan for.
Different venues operate on different payment rules. This year, CU Housing was fairly strict on asking for attendee counts and room assignments two weeks in advance. Then, as time gets closer, they got more flexible, and in the end told us that they will, in fact, only charge based on number of people actually attending - a huge win for us. Similarly with CU Dining, we had to go through great lengths to make sure that we would pay only for meals we ate. At CU, food had to be paid in advance, but accommodation and other costs are paid afterwards.
We have to guess how much will be spent on miscellaneous supplies once DebConf starts. We have to then decide how much to spend on extras such as the day trip or conference dinner, and how to sponsor those, if possible.
Debian (and thus DebConf) has money held in trust in many currencies in different accounts. Different sponsors will pay to different currencies, depending on what is convenient for them. This means that we need to consider fluctuating currency conversion rates, and decide if and when to transfer money. In general, DebConf hasn't transfer money just to make sure the DebConf-tagged money gets spent first. Some surpluses are best left in other countries to help pay future travel sponsorship since it is expensive to move money out of the country. Some conferences may result in a net movement of money from USA to Europe (more donors in one, more expenses in the other), some the other way around.
Travel sponsorship is in the form of reimbursement. These can take many months to have the documentation received from attendees and the reimbursements all sent out, and thus finalizing the budget and surplus for next year.
The overall principle we've tried to stick to is plan for a balanced budget in the worst case scenario, and then be happy when it ends up better than that.
At the DebConf&Debian BOF, someone mentioned how budgeting for any (free software) conference is hard. I think that's certainly true. Some of the registration fluctuations could be made easier by being less flexible, but DebConf wants to serve our attendees even if it is a bit of extra work for us. Each DebConf will have its own ways budgeting is difficult, but things have always work out, and usually with a bigger surplus than we were expecting.