Some food for thought...
First, you don't pay them and most of the time they have a lot of expenses to speak at your event. It doesn't matter if it's their company paying for it, it still cost them money. Yeah, there are benefits to speak at your event, but it takes a lot of time creating a educating and interesting talks with submitting the talks to events and creating the abstract slides, demos, code, dry run... There is a minimal chance that your audience went to all the same events as the speaker, and seen that talk yet, even for keynotes. When you pay your speaker, it's another story.