Much depends on the scarcity and demand for the bar or coin.
10 oz Engelhard you might be paying anything upto $600 - $700 depending on the type. Even some scarce PerthMint bars will have a premium. Most standard Perthmint bars sell for around $4 an oz above spot for 10oz bar. Hand poured bars generally sell for more.
The 2oz Kookaburrras have a low mintage and you might get $80 - %90 depending on the year and the spotprice.
The 1oz Lunar Dragon without the PrivyMark can fetch a good premium as there was only 300,000 minted.
Example where you can easily come unstuck. I pre-ordered the 1oz Lunar snakes thinking I needed to get in early. Paid $45 each for them. You can pick them up for under $40 now.
Thats just the nature of the market. Popular one day and no one wants it the next.