Sweetdaddy Rex
A lot, IF it's authentic ! GOOGLE it, and contact some Gibson distributors. ( Pawn shops will only give you half what it's worth).

Kab
Take it to a guitar repair man. Ask what it should be insured for. Divide by 2. Insurance value is twice replacement cost.

Mikey, just Mikey
Need the model number. Look inside the soundhole for the label.

OU812
Wow, I didn't even realize they made electric-acoustics way back then...learn something new every day. I'd call local shops and ask if they do certified appraisals.

Tony B
Are you sure its an electric acoustic? That would be VERY unusual. If f it really is custom made that would be VERY unusual too. What it was worth would be very difficult to estimate as it would be one of a kind. That, on it's own might not mean that is was worth a lot of money though.