Using the FLV format, my instructional videos use about 1mb per minute. This is a very high-resolution format (waaaay better than youtube) at a size of 480x360 pixels. This beats using SWF by a lot. I'll be interested to see how the new silverlight format does on this.
The discourse in these instructional videos uses approximately 130 words per minute, which for me comes to just under 10 sentences per minute.
At an average size of 4.1 characters per word, this makes one quarter of a standard 8 1/2 x 11" page of text each minute.
My videos average 14 words per sentence. The transcript contains 11% passive sentences, with a Flesch Reading Ease score of 80 and a Flesch-Kincaid Grade Level of 5.5.