I have several good Japanese Aritsugo and Suisin Yanagi style knives. You can literally shave the Salmon with these.
The knife that I reach for most of the time, to filet a Salmon, is an inexpensive F. Dick industrial use filet knife. This is a high carbon steel knife, with a curved blade, about 7 1/2 inches long. This is their model number 2417-21. It takes a razor edge, and holds it.
Mine is probably 15 years old or more. Here is the current offering:
http://www.mortytheknifeman.com/cart...ist.cfm?ID=413
Cost is listed as $19.20. It will cut almost as clean as one of my $800 + knives.