Some of the factors that affect the cost of iPhone screen replacement include the phone's model, whether or not the owner has a protection plan and whether there is any additional damage to the device. Another factor is the labor, as it is often cheaper to purchase a self-replacement kit.
The model of an iPhone is one of the biggest contributing factors to the overall cost of a screen replacement. Newer models usually cost more to replace due to the fact that there are fewer used parts available. Another major factor is the presence of any warranty or protection plan. For example, if an iPhone owner has active AppleCare coverage on the phone it reduces her total out-of-pocket cost because the plan absorbs a portion of the price. Replacing a damaged phone screen without any coverage means the owner must pay the entire cost of the procedure, including parts and labor.
While Apple performs screen replacements on all iPhone models, it is also possible for an owner to purchase a kit to complete the replacement herself. While this usually reduces the cost by removing the labor, it does increase the risk of accidentally damaging the phone's internal components. If Apple damages the phone during the screen replacement, it is liable for the damage.