How much is a golliwog cake plate worth?

Depends on the stamp and the plate numbers. Since the 40's, many people saved plate blocks from their sheets of stamps. Many of them are very common and can be purchased for about 5 times face value....

very expensive

It depends on the age and rarity of them. It's hard to say value with out age, or what they look like. :)

Infinite $$$$$$$$$

You will find this information by looking on the Grand Exchange.