lets see, are your inlays square or trapezoid? hardware gold or chrome? pickguard black or cream? headstock have that custom "diamond" inlay thing or does it say "les paul"? does it come with a hardcase?
many ways to differentiate a custom and a standard. You can check Swee Lee's website to get the rough price of the Epiphone Guitars, do note that Swee Lee's prices tend to be higher than regular prices so take Swee Lee's quoted prices with a pinch of salt.
From there, depending on the condition of your guitar, deduct accordingly. =)