Prix fixe means you can get three courses, but you must select those courses from a narrowed menu, and one price is charged for all three courses.
If you choose from the prix fixe menus at these restaurants you can get an appetizer on regular
DDP:
Le Cellier (lunch only)
Chefs de France
Captain's Grill
Kona Cafe
Nine Dragons
I hear there is another one somewhere. All these are one credit.
The things you have to remember are 1) you get the appetizer - but the entrees are not going to be the most expensive entrees. 2) No substitutions are allowed at all. I know Le Cellier will not even allow guests ordering from the prix fixe menu to substitute the side dishes.