$300Billion
Modern sovereigns unless in proof condition are generally only worth bullion. At the moment a full sovereign is worth about $400 US/CAD in gold.
Depends on the price of gold.
Australia uses the Australian Dollar.
Depends on the price of gold.
300 thousand
'The Red-Headed League' probably took place in 1890, and the gold sovereign was equivalent to 1 British pound at that time, which, in turn, was worth about $4 US. Inflation would make the sovereign's buying power about $100 in today's money.
The face value of a Sovereign is One Pound (GBP). Since all British Sovereign coinage is made from 22 carat gold, the actual value is very much higher. Athe very least, they are worth the bullion value of gold.
You have to explain what type of coin it is. Is it a British Sovereign? Canadian Maple Leaf? Australian Nugget? Etc. There are many modern bullion coins that feature Queen Elizabeth II on the obverse ("heads") but for modern bullion coins it is usually worth simply the precious metal content of them. On most bullion coins other than the British Sovereign it will say something like "One troy ounce pure gold" or "one half troy ounce pure gold" then you find out the spot price of gold and figure out how much gold is in your coin and figure it out from there.
8 grams
The second largest gold nugget in the world was found in Sovereign Hill. There is still gold there, although not as many people dig for gold as used to.
A sovereign contains roughly 0.2388 troy ounces of gold. At the current gold price, £1092.40 per troy ounce, you are looking at a value of around £260. Most gold sovereigns sell for around that on ebay. A half sovereign is obviously worth half of that: £130. If you mean purchasing power, I can't be sure but I believe a half sovereign would be the equivalent of around £75 by today's standards. I was looking myself for information on the purchasing power specifically, but the closest approximation I can find compares the value of a 1895 sovereign to £150 in 2007. I don't think it would have changed much.