Look at the gorgeous gold aureus in our picture. It comes from Sincona AG in Switzerland. It is such a beautiful piece, typical prices would be near 6000 Swiss francs, or about $6300 US dollars. If the coin were worn or damaged, prices would be much lower.
The $6000 figure gives you a decent idea of the value of such coins. That is a retail price. If you have one of these to sell to a coin dealer, expect a significantly lower price. The dealer is in business to make money, so a reasonable mark-up is required. More often such coins are consigned to auctions, wherein the seller, not the dealer, assumes the risk. The dealer charges a small fee for running the auction, and the seller gets fair market value for the coin.
But, speaking of risk, counterfeits abound.
Our second image of a Nero gold coin comes from Dr. Ilya Prokopov's Fake Reports which gives listings with images of fake ancient coins.
It is very important, then, to deal only with people you know and trust when buying or selling valuable coins. Don't get caught with an expensive fake!
For more details on ancient coins of Nero, click to this page by the Ancient Coin Club of Los Angeles.