Since the formation of the Open Source Geospatial Foundation (OSGEO) in 2006, there has been an annual FOSS4G event that brings together open source geospatial developers, users and supporters from around the world. The first event in Lausanne, Switzerland attracted 560 participants and last year's event in Boston, USA brought together over 1,100. The first FOSS4G event I attended was in 2007 in Victoria, British Columbia and what I remember most was the energy and collaborative spirit among all the participants. These are all volunteer initiated and run events and usually generate a positive financial return.
There are also regional FOSS4G events around the world. At this year's FOSS4GNA (FOSS4G North America) get together in St Louis, Michael Terner, who helped organize the Boston FOSS4G event, gave an overview of what is involved in organizing a FOSS4G event including a financial breakdown of the revenue and expenses for the Boston meeting.
This year's FOSS4G meetup will be held in Dar es Salaam, Tanzania, the second time the event has been held in Africa. The venue for the 2019 meeting has been decided - Bucharest, Romania. If you are interested in staging this event, Michael's breakdown of the revenue and costs will be very helpful in budgeting for such an event.
The Boston event generated about $1 million in revenue primarily from registrations, but 25% of the total was provided by sponsors. The space for the event was provided by Harvard University. It employed a professional conference organizer, but a lot of the work in making the event happen including finding sponsors came from volunteers. OSGEO provided about $70,000 as an advance to help get things started. The single biggest cost, nearly half a million dollars, was the venue and food. The next biggest costs were the social events and the audio-visual plus recording, each of which cost over a hundred thousand dollars. The Boston event generated about a $140,000 surplus for OSGEO.