Well, my formula would be to charge a rate where I can afford the time to do the client’s job right. Your description makes it sound like you really don’t care about the client’s content. That does not bode well for success.
Have you tried skimming the videos at double speed, or better, to listen for some of the best bits? JKL keys are your friend.
Whoever did the original content, probably shot them to a style or formula. If you watch one or two you can probably figure out what that is. For example, when I shoot these, I always use the same pattern of wide and tight shots for every “paragraph” or sub-topic of a lecture; Wide shot at the beginning, tighter, after the initial premise, where the speaker fills out more detail… then it pops between wide and tight and cut-aways and 2-shots for the bulk of the segment, with a tight shot on the overall summary statement. The next segment begins with a wide shot again.
If you skim my work with the sound off, you can tell by this pattern, regardless of the content, where each main idea stops and starts, and where the next one begins. Your preview would then take the form of making a montage of the summary statement sections, with perhaps one cut to the other, example sections. Or, if the presenter used an agenda slide, you’d probably feature that, plus a shot or two of the actual content.
Sometimes, the only way out, is through, and the only way to get thru a big job is to just bull your way through it, get into a zen flow with it. You might also divide the work between more than one person.
But I think your biggest problem may have been, you under-bid the job to begin with.