Cracked and broken slates are an obvious defect. When cracked and or broken slates are observed odds are other defects exist within the slate roof that require expensive corrective action by skilled contractors who work only with slate.
Removing and replacing a damaged slate can cost $75 EACH. OUCH, very expensive. Replacing the cooper valleys can run into the thousands as many slated on either side of the valley must be removed.
Should you be thinking about walking on a slate roof it is not at all a good idea. You will hear a cracking sound (the slates breaking under your weight) and if you have great reflexes you will then hear a scream (yours) as you go flying off the roof just waiting to hit the ground,