for render time... I went pretty easy in the past... I would just charge that out at $10/hr... easily making profit compared to the electricity needed to run the machines... but I also have seperate computers for rendering so it did not impact my hands on work time.
If rendering causes you to not be able to use the computer... charge a bit more.

I would also highlite that low charge as a way to make the clients feel good about the bill...

Mike Fitz
www.3dartz.com