The rarity of certain minted coins has a unique price depending on their surface quality and demand. These are price differently from bars (which I take it you mean minted bullion)
Bullion bars are measured in ounces (troy) 31.1035 grams and normally stamped by the refinery producing them. They come in many different sizes and the price of these changes everyday according to both the New York and London fixed price.
So if last week you checked the gold price on the internet as say US$800 that refers to the ounce I mentioned above on a minted (99.99) value. (highest quality) Other bars or small bullion issues have various stamps which also can affect their value. Different houses mint collectable stamps for a certain time which have an appeal to collectors.
Coins are different but are minted in a similar way and you could check ATS Bullion's website in London to see the rates in Gold Sovereigns and half sovereigns etc to get a good feel for the coin collectors prices. Older and rarer coins have a separate value as I said not necessarily based on their actual gold content but on their intrinsic value (what people will pay for them).
The most popular (bars) which are traded are quite small such as 1oz troy and also are minted into grams even up to 100 etc by other refineries. These are usually traded in quantities of ten or imperial dozens (12's) depending on preference.
Anything else below three or four nines Au (finesse of gold) is considered 'scrap'and has to be melted and assayed officially to determine it's value prior to sale or purchase.
Hope this helps