Some speakers are more willing to give a talk when you record them. On the other hand it depends on the type of conference. There are conferences which are sold

Some speakers are more willing to give a talk when you record them. On the other hand it depends on the type of conference. There are conferences which are sold out for years and do not need extra money. They want people to be able to see the talk athough they did not win a ticket in the lottery. And this is true for conferences which are very different. Google I/O, Apples WWDC and on the other hand the Chaos Communication Congress. All of them offer recordings for free.