In general, the best deals on gold coins are found on large retail websites that sell gold and silver coins and bars. The price per ounce decreases if you buy in bulk. Most retailers will offer you a discount for paying directly from your bank account, or sometimes even when using Bitcoin. Another simple principle is that buying in bulk is practically always cheaper.
Most dealers offer a lower premium per ounce (or whatever unit of weight you measure) when you buy a certain amount of ingots at a time. The main reason why gold cartridges aren't the cheapest way to buy gold is because they're not as popular, so private mints don't sell enough cartridges to bring the price down. Always use the method described above and you will never go wrong when searching for the cheapest gold products.