It's pretty cheap- $1.19 for three bars of Ivory soap, and food coloring if you want.
Put a bar on a microwave safe dish and nuke it for about 2-2.5 minutes. It's pretty neat to watch!
30 seconds in:
When it comes out you have a nice 'cloud' of flaky, crispy soap bits! Also, your whole house will smell like Ivory soap!
If you want to mold it, add just a tablespoon of warm water and mix it gently. That makes it more squishy. You can also mix in few drops of food coloring.
I was going to break out cookie cutters, etc, as well, but sadly the kiddos were bored within minutes :( Ah well. On to the next craft!