It varies greatly, and it always depends on the quality of work. For example, we're hiring an UH-mazing photographer. Extremely high quality stuff, and we're paying her $5K for eight hours and the rights to the photos (which will be sent to us on a DVD).
However, for us, photography was by and far the biggest splurge. We also joked that we would have hired her even if we got married at the courthouse because when do you ever have an excuse (or the funds) to have a photographer follow you around all day getting great photos of you and your family that you'll have for years to come?
Anyway, this is our photographer:
http://www.celimages.net/
For some people, photography isn't as important and therefore, don't want to spend as much.
I'd say anything under $6-7K is reasonable, if the photographer's work matches the price, and the shooting time would have to be at least 8 hours.