I would be worried about other ingredients or seasonings mixed in with the salmon burgers. I'm assuming they're not made entirely of salmon…?
Raw salmon also shouldn't be fed fresh, or so I found out. I'm pretty new at raw feeding myself, so this is something I checked recently about:
http://www.itsfortheanimals.com/must_cook_salmon_for_dogs.htm
That site says you should freeze it for 7 days, but I go for a full month. Presumably, the salmon burgers at Costco have already been frozen for some time, though.
We've been doing well with various kinds of fish that we purchase cheap from local Asian groceries. One of their favorites so far is whole, raw sardines. They're usually 2 to 2.5 ounces apiece, so it's easy to portion them out (mine get some kibble in the morning on raw days, so I just adjust the evening raw as necessary). I get 8 to a bag (approx. 1 pound) for just a little over $2.
We can also get salmon belly for pretty cheap, about $1.99 a pound. Again, that's frozen for a while.