I would be interested in this too ... I really need to sell mine but have been holding out because I feel sure I'll get a bad deal. (Not that I paid for them and would actually be losing money, but still.)
I'm not going to get into specific numbers, but I saw a return of about 60-65% on mine. Keep in mind that I sold mine on consignment at a local jeweler instead of outright selling them or pawning them. It took about 6 months. It's the timeframe he thought it would it take since I had a popular style and all the paperwork.
The best bet is to trade them in on something else - jewelers tend to give you a credit near the full value.
I have all the paperwork and one jeweler will give me about 40% cash or 50% store credit. I don't know if I can do better than that. We went to another jewelry store and they don't buy back jewelry. I think I'm going to try one in the town I work in to see what I can find out from them.
I haven't sold mine yet for the same reason. The offers I got from a few jewelers were low, like 30% of the original price. It wasn't worth it to me at that price so I'm holding on to them for now.
We had the same issue with H's old never worn wedding bands. I've recently taken to wearing the ladies band (on my right hand) because it is awesome. It was a $6000 band and we were offered $1000. H would rather have me wear it than take that offer.