Really depends on who's doing it. If it's Columbia Pictures..it's more than if it's a small software company doing an instructional video. For these I usually arrive at a "project rate" and include 2 redos in the package. One price for the finished product. Some stuff recently was a DVD for a real estate developer near Naples, Florida selling million-dollar homesites, and individual "tracks" for the different available builders. The developers segment was 5 min, and each builder got 3 min. Total running time was 20 min. I did about 13 min worth of VO. I quoted them 100.00 per voiced minute and gave them 2 "pickup sesions" The balked at the 100/min rate so I did it for 60.00/min. They only used one fix session. Be careful to specify a time limit. i.e. they need to use thier "fix" sessions within 90 days..or they will try and get you to do a new session like a year and a half later..
Here's a good one..we did the voice tracks for an instructional DVD for a small company that makes intercom systems for nursing homes. The DVD was for the installers to help them setup the system, and it also had a set of 'generic' voice tracks that they could use for programming the voice mail, and nurse-alert system etc. Example: "If you need medical assistance press..." then a series of numbers..."...1...2...3...star...pound" etc. We were unable to come to a price. They didn't know how much to pay..I had never done something like this before so i didn;t either. Eventually we agreed on a flat rate of 5% of the next system they installed using the DVD I did. They thought that was more than fair. Turned out to be for the Marriott Corp. HUGE nursing facility in Arkansas...intercom/alert system had something like 255 stations...thier rate to Marriott for wiring installation and programming was 475k! It was a good year...!
So the answer is..be flexible..do the job, and look for more work for the same people.