The most cost effective way to buy gold is to buy in bulk, so if you have the budget you should take this route.
Gold biscuits are smaller and you will be paying more for your gold, than you would with a larger bar. The purchase price of a gold biscuit will tend to be 2-4 percent higher than the value of the gold content you receive.
Try to study the market before buying, so that you can get a feel for when prices are lower, as you will want to avoid buying gold at peak prices.
The price of gold varies by the minute, but you can check the current price for gold here.