Conference venue costs - While I'd prefer we were at a hotel, to allow easier movement for attendees and speakers (and better nightlife/restaurant selection) other venues I've looked at in the area
start at roughly double what we're paying now. Oh, and then you have to add in A/V costs on top of that. Our conference center provides nice projection screens (huge) and nice mics as part of the rooms.
Catering - people need to eat. 99% of conference venues will not let you bring in random food, or in most cases even professional catering companies, unless they're on an approved list. Snacks/coffee/tea/etc throughout the day cost. Our food cost, including lunch, was north of $20/head. This is surprisingly cheap.
Power - some conference venues charge you for power access. $x/seat or something. We cut down on that this year compared to first year because few people seemed to use it the first years.
Wifi - our conference center has good wifi and it's free. One place I looked at said wifi access was $25/head/day. That's not a typo.
One of my goals was to have an event with non-local speakers. A barcamp is great, but... you end up seeing mostly the same people speak on the same topics. By having people from out of the area (state/region) speak, you'll be guaranteeing most attendees in the region won't have seen/met these speakers before. That costs something. Most of our speakers have been gracious with their time, but I wanted to at least cover some of their expenses in travel. With 20 speakers, and 8-10 with that sort of assistance, that's an additional cost.
I lost > $3k on the first event. ~$2k this time around, and would like to break even this year, or perhaps (gasp) make a small profit. A large profit would allow me to do regional versions of indieconf in different areas, bringing together a mix of local and non-local experts on various freelancing topics. That was the idea 2 years ago, but I can't commit to that until I know I won't keep losing money.
EDIT: oh yeah, insurance. The venue requires an insurance policy. For the minimum coverage, that's another... $600 or so.
EDIT 2: I would have probably broken even this year except for 2 things.
1) speaker dinner. Given the enormous effort many of our speakers put in (and travel) I like to have a low-key speaker dinner that's private for them to unwind. We have a few attendees that pay for a premium ticket to have a few dining hours for some one-on-one with our speakers. As fun as it's been, we may change that event this year.
2) After-party. This year instead of some ritzy club or venue, we had a jazz trio come in and had some catered snacks (veggie trays, etc). No alcohol, but I don't think anyone cared.
Without those two additional things, I think we'd have broken even this year on tickets and sponsorships. I'd rather not eliminate either one, as I think they each add something special for different groups.