well, gibsons are made in the USA, while epiphones are off being made in sweat shops in korea (I think... they are korean made right?) and also, gibson is like half machine built, and half hand built. because certain things are better done by hand, and others are more accurate and precise when done by machine (such as cutting out the shape of the body)

and hand built guitars always cost more, plus, things made in USA have higher labor costs to make them, and therefore higher final prices.

if you got an epiphone guitar and put some gibson pickups in it, you probably wouldn't be able to tell the difference (except you can't brag to your friends about having a gibson when most of them play epiphones, like I can)