I also HIGHLY recommend Manora - half the price of a regular restaurant, and delicious. The only problems are that it's always mobbed (the Swiss know it's the best deal in town) and it has department store hours, which for Luzern means early dinner Monday through Friday, lunch only on Saturdays, and no meals at all on Sundays.
Tibits is a vegetarian buffet; you pay by weight. It's in the Luzern train station, on the top floor (other eateries in the train station are on the bottom floor). it's open for lunch and dinner every day.
For Mürren, I had excellent meals at Hotel Bellevue and Hotel Blumenthal. A meal at these restaurants will cost at least CHF 40-50, but I felt it was worth every rappen.
Once you see Swiss restaurant prices, you'll want to buy some meals at supermarkets. In addition to saving money, it saves sightseeing time; if the weather's good at midday, you want to be out on a trail, not sitting in a restaurant. So, when in the Berner Oberland, I often just got trail mix from the Coop supermarket; I liked this because it doesn't spoil or get crushed in my day bag. They also have sandwiches, "picnic eggs" (precooked boiled eggs, dyed bright colors to avoid confusion with raw eggs), and other prepared foods.
There is a small Coop supermarket in Mürren and in Lauterbrunnen, and a larger one in Wengen (no supermarket in Gimmelwald). For Luzern, there are Migros and Coop supermarkets in town; Manor has a fancy supermarket in its ground floor; and there is a Coop and an Aperto in the train station. The Aperto isn't as nice as the Coop, but has longer hours and is open on Sundays when other supermarkets are closed.