There aren't many of them sold so there isn't really a market established, unless someone on this forum has sold a combo recently any value speculation would just be a guess.
Keep your eyes on eBay completed auctions, maybe you will get lucky and find one that sold. That will give you a more accurate value assessment than an enthusiast site.
From memory I think the Sunn combo amps don't seem to do as well as the separate head units, probably because of the versatility of separate amps and the weight of the combo cabs for shipping.
If you are trying to sell your amp, figure out what you would take for it and post that price in the classified section.