Salut Guillaume,
video recording is a real cost driver - you need expensive equipment (as of 1.000 USD / day) and a lot of staff if you want to make it professionally.
Sometimes you are lucky and video production is offered by the venue or a video producer is providing sponsorship but otherwise it is really a question of costs. Often the compromise is then to only record the main auditorium and skip the rest.
What you can do is to bring your own camera and mic for a Google Hangout session and recording, as I did for the MediaWiki Stakeholder session. This won't be very professional, though.
/Manuel