Captions are helpful for those with auditory impairments, and audio (spoken) descriptions are very helpful as video descriptors. However, sign language adds much more depth to the media; it has expression, personality, and dynamics that simply aren't possible with captions or audio descriptions. This level of compliance is considered Level AAA (the highest level in the WCAG 2.0 specs).
Say you're doing a webcast, and you're providing a signer. They could appear in a small box on the bottom of the screen. Easy breezy!
You're involved in a corporate event, presentation, or speech. An interpreter could be at the corner of the stage. If it's recorded, ensure the camera operator captures the interpreter in the shot.
There are many interpreters that are available for both onsite and remote (e.g. webinar, etc) presentations. Many use technology such as video remote interpreting that facilitates this service. As for costs - it's not cheap. Most VRI services will run between $2-4 a minute, but it's well worth it to make the content accessible for those experiencing impairment.