2 years ago when I last went to Nationals (in Reno), I think it was $5 for a coin to open the locker and an additional $5 each time you wanted to open that locker. I had the VIP package which gave me unlimited access to a locker, so my pricing information is based on what I saw on the coin machine and what I had heard from others that used lockers.
If you are using the travel 3 ball totes you should be able to get 2 of them into the locker as long as they are vertical. If you are using a bulkier 3 ball bag, then you will get 1 of those and a 2 ball tote.