I just switched to Sea Pet Gold. To see whether you are getting your money's worth, note how much EPA+DHA you are getting per serving (note what a serving size is - not the same for every brand!). If you really want to crunch numbers then divide that up by what's in the bottle and what you are paying, and you can compare apples to apples that way.
Look for fish body oil of any kind, and guaranteed free of heavy metal contaminants. Usually that means fished in deep cold water.
Check for packaging that preserves the n-3s as much as possible. n-3s are sensitive to light and oxygen. Green or clear bottles are a waste of money.
The Sea Pet Gold gives me the best bang for my buck. Just 1/4 teaspoon provides a therapeutic level of n-3s for my each dog in my gang, except my chinese crested who just gets a drop.