Ballpeen hammer does do the job but takes a lot of arm power and you can tire out easy if you have a lot to do. Also you can't be 100% sure that your hits broke every platter. For what you are talking about I don't think its a very good option. THe best option for what your talking about I think would be the log splitter that people talk about on here. You can crush 2-3 drives at a time all for the flick of a button.

To maximize your money you could take the boards off split them in half with the splitter than separate the aluminum and stainless and magnets if you wanted to. If you didn't want to fool with it take the boards off split the drives and then sell it as breakage. If you wanted to give them proof take the serial numbers of the drives down when you pick them up then take pictures of the drive crushed with the serial number visable.