Depends on the property, but used to be less than $ 15 at some places.

Many Hyatt properties in the US do not have a breakfast buffet, so they allow you to choose anything from the menu, or, as pointed out already, subtract a certain amount from any meal of the menu.

Some properties, however, will only offer you a choice of some items that make up a typical full american breakfast.

In Europe, at least in many countries, it is less common to order meals for breakfast, so you should always get the full buffet.

There has never been a difference between continental and full breakfast for Diamond members in the past in Europe, however, some US properties used to charge some $$$ in the past, when a guest intended to consume items from the full breakfast buffet and not only the continental breakfast buffet.