Any resonable restaurants in Cape Cod
I'll start with this assumption -- that food that is fairly priced but may be expensive is not what you'd like.
Then here are some places that serve basic New England style food to the locals. Now remember, the locals may not have sophisticated palates, but they do like simply-prepared seafood that is fresh and is inexpensive:
The Dolphin in Barnstable Village
Wimpy's in Osterville
Kettle Ho in Cotuit (especially for the stuffed quahog or fish special)