I always think that you are paying more because of the brand name, my husband takes Cod Liver Oil & Glucosamine, Chondroitin & MSM we found a site which is good value for money, its :-www.purelyheathdirect.co.uk and the postage and packaging is free. :-)
You will find that Cod Liver Oil comes in different strengths in capsule form and some is purer than others (like olive oil). You normally only get what you pay for. The cheaper ones are usually not as high strength as the dearer ones and you may find that they say to take 2-3 per day instead of just one. You need to work it pro rata.