Well....here's what I can tell ya
One of my friends is a doctor and he's really big on nutrition. He's asked a lot of his older patients and people in good shape what they've done in their life to sustain their great condition....and most the answers involve exercise, eating right and taking good supplements. He was fascinated with the many supplements on the market and quickly discovered that like most business related things, the primary goal of these supplement companies is to make profit....accordingly, they understand that consumers are just like people looking to fill their car's gas tank: the lower price-per-gallon gets the business....and that pretty much means the lower the price, the greater the perceived value and the more bottles & volume moved.
What he found is that you get what you pay for IN MOST CASES. When you get 1,000 capsules for $9.48 you probably aren't getting anything very potent, real or effective. He told me point-blank that Costco stuff is straight-up diluted, cut and compromised in quality substantially. Now, I ask myself: did he take these Kirkland products and actually test them? On what basis does he justify that claim? I never really got around to interogating him....but in the course of my knowing this guy (and seeing some of the remarkable surgeries he's done)...I've come to regard his words as solid.
It's a tough call. Just last night a nutritionist told me that if you eat fish 3x per week then you don't need to supplement with fish oil....that we get all we need in the food we consume. So who's right, who's wrong? In the end, you can find research, data and testimonials supporting all sorts of arguements. And what do we know....what we read? And the writers of such material, what do they know...what they research? And aren't these the same conclusive researchers who told us that Vitamin E was the wonder/miracle supplement to prevent heart-attacks AND NOW WE COME TO UNDERSTAND that taking more then 400iu of Vitamin E is linked to a greater incidence of heart-disease....so somehow all that concrete research & clinical studies somehow got it all backwards?!?!?!?!?!? Wtf?
So I don't know what to tell ya.....seems like everyone is on the fish-oil kick, whether it's to maintain eye-sight, brain function, anti-cancer or what-not. Research says one thing, then a decade later it flip-flops and says another. I wouldn't be the least bit surprised to find that the benefits of fish oil are overshadowed based on the damage from mercury in the fish.
So in the end....do what I do: go get some of the high quality stuff and mix it in with your Kirkland stuff until you deplete the kirkland inventory, don't throw it out. And in all fairness, a lot of companies take the same cheap-stuff and tout it as high quality; it's all marketing, claims and presentation. In the end, buyer beware.