Personally, I quote different prices based on pixel count and rights agreement. My prices range from the lowest for non-commercial reproduction rights; with a significant rise for commercial reproduction-of-image non-copyright agreements; and are highest for full exclusive copyright agreements. I'd imagine this is the norm.

I also agree with RobA that visual artists should not undervalue their work - but at the same time I think it is important that new artists are able and allowed to do their first few jobs as cheaply as they want while they are establishing themselves and putting together a portfolio.