Nick,
Funny you should ask that question, I have pondered the same question for a while, - lets say a tonne is 1/2 cubic meter or thereabouts then I have seen spots produce 2-3 grams per cubic meter and some spots only 0.5 grams per cubic meter , having said that the benchmark in Alaska for commercial mining is about 0.25 grams per cubic yard or about 10-12 USD, On average you could say Oallen is good for about 1-2 gram per cubic meter I reckon. Americans would think they are in heaven if they came out here but it wouldn't happen due to the green tint in our politics and the fact that the shoalhaven is a domestic water source.
Cheers
Barry