There is no standard answer. Could be as low as 3 grams a yard to as high as a thousand ounces per ton.
The point of mining is to make a profit. Before you can figure out if a deposit might be profitable you need to determine the nature and extent of the deposit.
The nature of the deposit is about what type of material needs to be mined to obtain ore. Hard rock is generally more expensive and requires more investment. Placer can be less expensive to mine. If the ore needs extensive processing and a lot of water or power imported costs go up. If it's hard to get to and mine it changes the nature of the deposit. Transportation, labor, lodging, fuel, equipment, repairs and maintenance and throughput need to be calculated to best fit the deposit. Marketing, depreciation, permitting and taxes need to be factored in.
The extent of the deposit is just as important. Whether hardrock or placer you will need to drill and profile the deposit so you know where the pay grades are and how much mineral you can reasonably recover from the deposit.
That's the simple version. Do the math on the recoverable ore in place minus the cost to extract, refine and market it and you will discover what market price you will need to get for your finished product to break even. From there you need to see if there is enough headroom on the market price to provide you with a reasonable profit over the period of time it will take to mine, process and market the deposit.
If you are thinking of placer mining studying the
Cost Estimation Handbook for Small Placer Mines would be a good starting point. It's only 100 pages but it's a good overview of what is required to check a placer deposit for potential profit by first discovering your cost basis.
Hope that helps!
Heavy Pans