The differences between Epiphone and Gibson are now very much the same as the differences between a Squire and a Fender.
Epiphone is Gibson's entry-level guitar. It's a way to introduce someone that's more on a budget to a Gibson through Epiphone.
So when you're dealing with Epihones, you're getting guitars that are made overseas not necessarily equal quality materials. They try to maintain a certain level of quality with Epiphone. The reasons why Gibsons are more expensive are generally materials used, and here they are made. They can keep production costs down by making them overseas.
It wasn't always like that. Epiphone, at one point in time were made side-by-side with Gibson in Kalamazoo, Michigan, and Epiphone was designated as their export model. Epiphones were made to go overseas, and Gibson stayed here. At some point during the transition of the company, they decided not to have the two compete with each other in the US market. At some point in time, in the company's evolution, Epiphone was designated an entry-level thing rather than having two lines doing the same thing, top-quality and price point. They could make the Epiphone a little more cost-effective, charge less money for it, and keep Gibson as their jewel.