not too sure about the price
but you should look for:
- shutter count (how many pics the camera has taken...the lower the better)
- dust in the body/lens (take a few pics of a light background at the smallest apperature and see if you see any)
- physical damage to the body or lens (scratches, dings, dents, etc)
- any funny sounds when autofocusing, when taking a pic, or any rattles
- choose the smallest apperature and press the depth of field button and look at the apperature blades to see if there's any damage
- make sure zoom and autofocusing rings operate smoothly
- test flash
that's about all i can think of now