The lockers at the rides that don't let you carry anything on (Forbidden Journey, Hulk, etc.) are free for a limited time - usually 2-3 times the posted ride wait time. The bigger lockers at the front of the park have an all day fee.
It depends on how long the Express Line is and how much time the locker is good for. It is stated on the locker screen that they are single entry only, so if you ride multiple times and go over your time you will get charged.