I have a 18k Cartier Gold Chain 36 inches and 68.5 grams in weight Vintage how much do any one think it's worth? It is 2.4 ounces

Using the current spot price of gold according to kitco.com ($992.20/ozt) you use this formula to calculate the value of your jewelry:
grams (68.5) X purity (0.750 for 18 karat) divided by 31.1 X spot price of gold ($992.20) = $1,639.04. THE SPOT PRICE CHANGES MINUTE BY MINUTE, SO YOU HAVE TO WATCH IT. The price you get using this formula is the "scrap" value of the actual gold content, not necessarily the value if you consider the uniqueness or rarity of the particular piece of jewelry. There are always buyers of scrap gold searching for sales on eBay. Good luck, hope this helps.