Actually, you HAVE to closed caption as part of pretty much any broadcast pre-license agreement, even if you are an independent producer of documentary. Even my low budget work has closed captioning. It isn't that bad for expensive, either (about $1500 per hour, unless you're paying for transcription as well, which will put you up to about $2500, which is still relatively inexpensive in terms of production costs). So don't be dissing us documentary-makers, we almost always closed caption.
Oh, yes, and as a budget line item, technically we pay for it as part of the cost of production, which is funded by a combination of broadcast pre-license, tax credit (that would be producer equity), and potentially other funding such as the Canadian Television Fund.
As to acquisitions (which a foreign property like V of D), it may be different, and of course a series may be expensive to CC, inclusive of transcription.