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Here is something you may use for your negotiation:

The G1 in Yamaha's hierarchy then was similar to GC1 today. Find out how much is the street price (not the list price) of a GC 1, then use Larry Fine's Depreciation Schedule for Pianos (The Piano Book, 4th ed., page 203.) as reference.

 

According to that schedule, a 25 yr old non-Steinway grand should worth 31% (below average condition), 34% (average) and 37% (above average) of the street price of today.

 

Some one here might have bought a GC1 recently and be willing share the price with you.

 

Larry Fine's schedule is not accurate, but it can help your negotiation.

Most of the dealers should have that book.  You may also find it in your local libraries.

 

Also, a piano this old is at the end of its life, at least it is opinion of most of the piano buyers.  In other words, when you are ready to sell it, most of the buyers may turn their back to you, thus your resale value would drop sharply.

 

BTW, the G line had nice pianos.  G3 and C3 were almost the same size.  Many prefered G3 over C3, tho the latter were more expensive, and better materials were used.