We've heard it all -- the dog stepped on it, sunglasses were left on the keyboard when the computer was closed, it fell "just a few feet," and many more.
Screen damage is pretty common, and we've done plenty of them. For non-Retina screens (mostly pre-2012 computers), there are two parts -- the glass and the digitizer. As long as the digitizer is not damaged it is relatively inexpensive to replace just the glass.
With Retina screens, in typical Apple fashion the two parts are fused together, making it much more difficult to replace just one of them. It can be done, but it's generally not worth it.
Different years, models and sizes are priced by demand and complexity, so they are all different. You can get an estimate for your model on the order page, or using the menu above.