Dry Ice is so cheap. Just call around to your grocery stores and see if they have it. It normally sells for 25 Cents for the amount you'll need. I have a little kill chamber that I've made out of old 3 lb cottage cheese tubs. I've only killed one at a time, but every time I do it, it's so much easier than whacking. Then whatever amount of dry ice you have left over, you can play with it.

I've had to whack rats and mice before... It's not as easy as it sounds. Go with Co2 or Dry Ice.