Watch the show.  There are hundreds of episodes.  They repeatedly pay right about half of current retail value.  Maybe a bit less if the item will take up lots of floor/wall space, or it won't sell for years, or it will cost additional money to prepare the item for sale, or there are other risks involved.  Or maybe a bit more if the item is small, easy to sell, and there's few risks to the shop.  Gold and silver are the best examples.