In relation to the total costs of a conference, probably less than you think (see this sample breakdown of costs.)
Participants come to a conference to share with others what they know and hear what others have to say, in short to communicate. To do so, they prefer to speak and hear their own language or at least a language they feel comfortable with. Your delegates are making long and costly trips to attend your event. Make sure they are able to follow proceedings by hiring a CIAP Consultant Interpreter.