I have a really cheap $5 model from Staples that I love. It runs on batteries - no power cord option. I've had this one for over 3 years now and it works great on everything I've thrown at it - prismacolors, regular pencils, chalk pencils. I figure for only $5, I'm not going to be too upset when it breaks; in fact, this year I picked up another one at the back-to-school sales just in case.
I should mention, I think the price on this when it's not back to school time is around $10. But if you buy now, it costs less and is available in better colors than standard office fake-woodgrain.
~~Cheryl, with a "C"