Nope, no charge to use them.

They use a key you can take with you (unless they've changed within the past year). Well, they're safe... as a safe. (We've had no issues, although I assume someone on property holds a master key to the room safes

).
If you head over to the BC/YC FAQ thread, I'm sure they've got a picture of it on the first page.
My only issue with the safes at the BC/YC are -- since they are on the smaller side -- the bit where the key is inserted, where the bolt resides, sticks into the safe itself so you have to be careful where you place electronics. With an expert-level Tetris degree, I've managed to fit in the following with no room to spare: 2 smartphones (Galaxy S3's to give you an idea of size), an old Kindle in a leather case, 2 wallets, a Nintendo DS and a Nikon DSLR with lens attached.