I'm doing this on memory...
Last September I think we paid $7 to get a small locker and when you bring back the key, you get two bucks back. We found the small locker adequate to store basics plus a camcorder. But note that there were only DW and me. I think the big lockers were three bucks more. You can, insted of getting your deposit back, choose a commemorative mug (non-refillable). It's a pretty cheap keepsake.
Note on the towels: DON'T DO IT. They charge you two bucks for a small piece of thread. Bring your own. The lockers are a bargain for what they provide. The towel rental is a joke.
Pat