I'd take a look at Gunbroker for the best price estimate, but as pointed out, you aren't going to get top dollar from the dealers. I'm always amazed at guys walking around and expecting dealers to pay retail for their guns or getting upset when they receive what they think is a "lowball" offer. The guys sitting behind the tables are there to make money.
If someone wants something close to top dollar at a gun show, he or she will have to put a for sale sticker on it and walk around where so other attendees can see it. Sometimes it works even without a sign. I bought a pre-64 Winchester at a show a couple years ago and was offered $250 more than I paid for it before I got to the door. It had been sitting at a table cluttered with AR-15s and AKs and was I suspect just over looked by prospective customers who are not generally the AR or AK type.
Otherwise the only reason to sell a gun at a gunshow is to turn it into quick cash. The same applies with a local gun shop. They'll offer you 50%-60% of their expected retail sales price. They have to cover the overhead (rent, utilities, staff, insurance) and they the money they pay you tied up in the gun until it sells. That all adds up and a 100% margin does not equal anything close to 100% profit.
Sellers can get around 80% of the market value by selling through a local gun shop on consignment. With that approach the shop doesn't have to tie up it's money in the firearm until it sells. It's just taking a commission for the space to keep the firearm in the rack, show it, and sell it.
Not all shops are interested in that approach as a consignment gun is competing with their new and used inventory, but if you price it right so it will sell quickly instead of sit there for months or years, the odds are good that a shop will work with you.
The other option is to list it on-line. That will get you 100% of market value, minus sales and listing fees and minus fees to an FFL to ship it to another FFL. Not all FFLs will receive firearms from a non FFL as that can create or lead to a number of problems. Depending on the type of handgun, the shipping cost for an FFL can also be significantly lower, and in general a business pays less to ship with a business account than a private individual. So at the end of the day, your shipping cost as a seller ends up being significantly higher - and one way or the other than comes out of the final price you get. Those accumulated costs and fees can make a consignment sale more attractive than an on-line sale.