It really all depends on the condition of the box itself and the styrafoam, since that is primarily what you are paying for. If it is MISB in immaculate condition, I wouldn't pay more than $700 for it, and the price would drop steeply from there based on how beat up it is. I assume you are talking about the US issue boxset and not one of the foreign ones. But that is just my opinion. Crazily enough, someone had an AFA'd one on ebay for $10,000 a while back. It had high grades, but nothing to warrant that kind of price.
If you go buy the price guide it is only worth about $500.00 sealed and about $200.00 MIB and that is if it is complete with paper work and in great shape.
thanks, just got offered one, not sure if it is sealed or not though wanted to know if it was worth it