24k is pure gold. 18k is 18/24ths or 75% gold, 12k is 12/24ths or 50% gold, etc. So, you could expect to get about that percentage of whatever the 24k price is.
However, there are two gold prices.
The price you pay as a retail consumer and the price they pay to buy it back. The price you sell it for will be less than the retail gold price. If someone offers you 90% of the retail gold price you are getting a really good deal. Jump on it.
I don't work in gold, and I re-melt my silver scrap, so I don't know the current percentages you could reasonably expect.
Also, there is solid gold, gold plate, and gold-filled. Solid gold is what the above prices are talking about.
Gold plate is a very thin layer of gold over some lesser metal. If you're lucky it's silver. But it's probably not. And when I say very thin, I mean so very thin that you won't get much for it.
Gold filled is a thicker layer (5% to 10% of the overall thickness of the metal), so only 5% to 10% of the weight of the item would actually be gold. And, of course, if it's only 18k gold filled, then we're talking 75% of that already much smaller number...
And get multiple bids...