Frugal

The trick is to add a lot of walnuts and not a lot of salmon. One piece of salmon can provide two servings this way. I consider it frugal because it is less than half the cost of fast food.

It might not hold true now (Covid and post-Covid price gouging has been a PITA), but dollar stores sometimes have cheap cans of mackeral. Grocery stores do too--basically, go to the tuna aisle, but take a gander at other types of canned fish and see what's up. Sometimes you can get a deal. It's not salmon, obviously (...unless it is, lol) but it's protein.

I have also found if you're in a city with an ethnic grocery store, they often have cheap tilapia, basa, or swai. Basa/swai are an asian type of catfish that aren't allowed to be labeled as catfish due to protecting the local catfish industry, but they taste much better than local catfish (they don't have the "muck" or earthy taste.) (Speaking as a midwesterner. You coastal peeps might get better catfish locally.)