I'm going to assume that your weight is in avoirdupois ounces, as that's the most common. But gold is sold in troy ounces which are not exactly the same. A troy ounce is a little smaller than an avoirdupois ounce.
6 avoirdupois ounces = 5.46875 troy ounces.
18k gold is only 75% pure gold, so we reduce the above by 25% which comes to 4.1 troy ounces of pure gold. Right this second, gold is at $1431 per ounce, so 4.1 ounces would be $5867 worth of pure gold.
Now, you have to remember that your scale is probably not perfect because you gave a round number for the weight, and every tiny fraction counts. And nobody will pay you the full price of gold, since they have to make a profit, especially when the gold is not pure already. So I would expect to get about $5300 plus or minus to allow for inaccuracy of your scale.
The easiest way to find out what you could get is to call a couple jewelry shops and coin shops and ask what they are paying for 18k gold. They will probably give you a price per gram or a price per pennyweight.
There are 31.1 grams in a troy ounce, and 20 pennyweight per troy ounce. So you can adjust between them if you need to.