Magic Slates are very simple toys. They are just a square of black wax affixed to a piece of cardboard with a clear page attached. The child uses a tool to draw on the clear page and when they wish to erase it, they simply raise the clear page and start fresh.
The story goes that in the early 1920's, a man approached the owner of a small printing plant to sell the rights to his invention. It was a simple homemade device made of waxed cardboard and tissue. The owner of the printing plant, R. A. Watkins, told the man he wished to sleep on the proposal. During that night, the inventor was arrested and called Watkins offering him the rights to his invention in exchange for bail. Watkins paid the man's bail, got a US patent, US and international rights and made millions.
What's it worth? Take a look at this Magic Slate price guide: sold listings for a value indication.