There is actually something more to the premium brands than the concentration of DHA and EPA; in fact, there are inexpensive brands that are very concentrated, and premium brands that are not. I used to just take the cheap stuff because I couldn't find any substantive difference between the cheap stuff and the good stuff. I did hear a LOT of stories from people who got no results until they switched to the premium brands, but I dismissed it as placebo or something. However, I am now sold on the premium brands.
The explanation is a little convoluted, but it is good to know, and this is one of the most important supplements you can take, so it is worth winding through.
The EFAs are present in fish in triglyceride form. In processing, the fish oil is treated with ethanol to extract the oil; the resulting oil is in ethyl ester form.
Triglycerides form is pretty stable; ethyl ester form is not. EFAs are very volatile in general and easily oxidized. Once they are oxidized, they are no longer healthy oils, they are the dreaded trans fats.
The premium brands go through additional processing to put the EFAs back into the original, stable, triglyceride form. They may also undergo other processing to remove mercury and other pollutants, to remove the fishy smell / taste, etc. This is where the price is coming from.
This information is not really getting much coverage, virtually none until the last year or so. The manufacturers of ethyl ester form are defending their products, but what do you expect them to do? In fact the makers of the prescription fish oil, Lovaza or whatever, actually make a point of saying their product is the inferior ethyl ester form in their advertisements.
I think part of this is, the drug company running the ad knows many people have no idea what "ethyl esters" means, but they will assume that the prescription form is in some way superior to the Sam's Club form because it contains "ethyl esters." In the mean time, they (or actually their insurances companies, i.e., you and me) are paying way WAY way more than the premium brands cost for the prescription brand; they drug companies are gouging and they don't even have the class to gouge on the good stuff.
I suspect another part of this is that the premium brand makers are hesitant to brag on their triglyceride form oil, because people know that blood triglycerides are a bad thing, and they will be confused and think that triglyceride form oil will raise their blood triglycerides. Ironically, lowering blood triglycerides is one of the things fish oil is most effective at, and it is the triglyceride (TG) form, not the ethyl ester (EE) form, that is most effective.