Lester G
Just to be clear, it's not a Gibson, it's an Epiphone. There's a big difference. The serial number isn't the key - you need the model number. There are lots of Epi LPs out there, some selling new for as little as $170. Used gear sells for about half of its new selling price. Your best bet is to determine the specific model, find out what it sells for new, and ask for half of that. Good luck.

ImpulseGuitar
Best thing to do is look up some of the same guitars on Ebay. That will give you a good idea on what they go for.