Typically speaking, the bigger the size of the item, the more economical it will work out to be per gram/ Oz. That doesn’t mean that spending the entire budget on the biggest piece you can afford is necessarily the best way to go though. That depends on the individual and their strategy/ reason for buying.
How you buy it is the same way you will sell it. If the plan is to lock up a large sum of money and unlock it all at the same time, the bigger the better is probably what they want to do, however, understanding the difference in price by buying in smaller denominations can help the customer to decide this if they’re not sure.
For example, smaller bars of gold (1,5,10 grams or fractional coins) have a higher rate per gram compared to 1 oz cast bar, the smaller they are. So a large investor would probably be unwise to buy a large amount in smaller bars unless they are wanting tradability as there’s a reasonable difference in the price.
However the difference in buying 1 x 10oz cast bar of gold (currently a little over $27,100) as opposed to 10 x 1oz bar for example, is roughly only between $20 and $25 and ounce, meaning a difference of $200 – $250, which while not an insignificant amount of money, compared with the overall size of the investment, is a fairly small number and they may and often see this as a small price to pay for the flexibility of being able to sell in in potentially 10 separate portions rather than being tied to selling it all at the same time.
Given the customer is likely buying something of that value as a long term investment, they are obviously of the belief its value will grow considerably by the time they go to sell it.
The same logic will apply to all sizes and colors. For example, 1 oz silver coins are more per oz/ gram than 10 oz or 1 kg but offer flexibility and for some cases, tradability. A 1kg bar is a little over 32 ounces.
If you compare 10 oz bars with 1kg bars, the price of 3 x 10 oz is is pretty much the same or very close, so buying 1kg will give you an extra 2 ounces for roughly the same price, but the 3 x 10 oz has a little more flexibility of being able to sell them in 3 separate portions.
There is no right or wrong answer here, it’s a matter of opinion and situation. By helping the customer understand this, you can help them with buying in a way that suits them best. It is also worth being mindful of what’s in stock and what’s available too though.