It certainly varies by place. In San Diego, the typical Restaurant Week menus cost $30, and include an appetizer, entree, and dessert. Most of the places where I go for restaurant week normally charge something like $21-28 for an entree, $7-12 for appetizers, and $6-8 for dessert. Some places downsize the entrees, but most don't seem to do that. So it really comes down to whether or not you would have ordered both an appetizer and a dessert. If you would not have, then Restaurant Week probably offers very minor savings at the expense of a reduced set of choices. But on the Restaurant Week menu, you basically get either a free dessert or a free appetizer as part of the promotion.

And nobody is forcing you to order the Restaurant Week menu - you are perfectly entitled to just get whatever you might want off of the normal menu.