It's worth exactly whatever somebody is willing to pay. About the only actual barometer of current used gear value is completed eBay auctions, and probably because of situations such as your own, the market for the big (four output tube 1000 series, 2000S, 350L, 350B type) Sunn tube amps like yours has been tanking as of late.
Not long ago an amp like yours would bring $1000+, but here's what I've seen lately;
2000S - $710 on 4/1
2000S - $685 on 4/12
Coliseum PA - $472.03 on 5/13
It's certainly a buyers market, but if you can hang on I'm pretty sure the market
will once again spike upwards.
After all, they
are fantastic amps, comparable new "boutique" hand wired amps still bring stupid money, and they
sure aren't making any new 350B's, 2000S's, etc. any more.
Good Luck!