There are likely other variats between models, but the majority of $$ differences is in the name. The selection of wood is sometimes a factor. I've held a Gibson LP and an Epi LP, I found the EPI to be a lot lighter. The Gibson was made with older growth wood - the grain is tighter, hence the wood is heavier - which they believe helps add to the sustain. The selection of pickups may not be pleasing either, but they are easy to change down the road, should you like a better tone.
Overall, I like what I've seen recently from Epiphone.