Bullion coins are priced with the market. The lowest people USUALLY pay is the spot price of the metal - but that can change with supply & demand. Its no different than anything else you want to shop & compare. Google bullion dealers. Look on their gold bullion page. If they don't list both their buy and sell price then call and ask them what their BID is on xxxx coin that you have. Shop & Compare!
There are several sites offering advice on this, but I think 24carat.co.uk seems to be the most honest and to offer the fullest advice. They also provide useful links to pages where you can learn more about how to value a coin.