A spoken corpus is about ten times as costly as a written corpus of the same size.
This is only the workload of verbatim transcription, not to speak many other features common to speech, like, overlaps, pauses, simutaneous speaking, paralinguistic features, and prosodic features.
Transcribing Oral Discourse A Survey http://www.corpus4u.com/forum_view.asp?forum_id=53&view_id=263