I did the Fantasmic dining package this year at
Disneyland California at Blue Bayou and I have also eaten just a meal at Blue Bayou. The Fantasmic package is a set menu, set price, a starter, main course and dessert, as well as a ticket for the reserved viewing area and at Blue Bayou you also get a foam cushion to sit on while watching Fantasmic.
If you just go to eat at Blue Bayou, you choose from an Al A Carte menu. This means each course is priced separately. You can just have a main course if you want and yes this will be cheaper than the Fantasmic package. You could also have all 3 courses and drinks and sides which could work out more expensive than the dining package.