The lockers are $7, but you get $2 back when you return the key, so they are actually $5 a day.
The lockers come in 2 different sizes. The big ones are about a third the size of normal high-school locker (about the same width, take the height and divide it into thrids). The small ones are about half the size of the others. MGM only has the smaller ones. It was a tight fit for a large LLBean bookbag that was full with clothes to fit, but with a few helping hands, we got that closed.
They don't fill too quickly and I really don't think that they sell out since there are quite a few of them.
Hope this helps and you have a great trip!!