Gibson used to have a on-line query of their guitars value. You place the model, the year and a "rough guide line" how much the market value.
Now they recommend you get Vintage Guitar Magazines' George Guhn's Blue Book Guitar Book.
Over 10 years ago, a '77 Les Paul Custom was worth $1200 - $2000. Even if its built in Norlin where some vintage aficionados claimed those were junk while other say they got a bad rapped because Gibson started to cut corners at time using pancaked Mahogany bodies and the tenon neck did not go into the pickup cavities like their vintage models.
Some made a big deal about it. Some say, can you really hear the difference?
Sorry I did not answer your question completely. I don't know what your 78 Gibson Les Paul Custom is worth Today.