I think the real issue is being sure the battery has the same or better cold cranking amps (short circuit current), amp-hour capacity and reserve capacity as the original. Details of these terms can be found on battery manufacturers' web sites or by googling. Sometimes in warmer regions of the country stores sell smaller capacity batteries for a given car - which are cheaper, but if you travel much to cold regions you could be out of luck even tho the battery is not defective - also a possible problem if you sometimes use accessories in the car such as lights, a high-powered stereo etc, when the car is not running.

That being said, prices vary for the same performance. A quick way to compare one battery with another of similar dimensions is to pick it up or weigh it. The better one will have more lead and sulfuric acid in it and be heavier. I bet that your OEM battery though overpriced is one of the heavier ones you find.