Depending on the length of the script..ie. 5 minutes vs. 1 hour...some narrators charge a flat fee for the first hour (about $250hr depending on your demographic area) with incremental fees for any time over the estimated amount. Of course a 1 hour long course script may take the narrator 2 hours to narrate into final form. I've been producing videos for years and you can easily spend a half hour on a 30 second script.
In the training/e-learning/corporate market you should pay your VO person at least $250 for the first hour, more if a special talent is required. That's just for voicing, mind you. I also have my own home studio (where I also do work for broadcast) and I don't think it's unreasonable for such VO folks to charge something for the post-production and file management activities. At that point you're asking for someone who can voice AND edit. Two services, two charges. Not everyone can do that.
Since this became a general discussion on rate structures, I thought I would chime in that another method exists. You can take a look at the Narrator Files (www.narratorfiles.com). The rate structure is based on a flat $20 per-page fee, the talent is professional and turnaround is 2-3 business days.
I've found it best to determine the finished number of minutes of narration by taking the number of words divided by 160, which is the average rate of narration words per minute.
To calculate finished minutes, I divide the script words by 170wpm. The rate per finished minute for the talents range from $10 to $50 depending on their skill and popularity. The session minimum is generally either $150 or $300 -- as requested by the talent to accommodate their studio and/or editing fees. Most have a home studio and are non-union.