How much would 400 weight of Gold actually have been worth?

With respect to the well-intentioned conjectures others have proposed, the measurement Captain Haddock uses in the movie isn’t “400 weight”, it’s “four hundredweight”. A hundredweight is, indeed, a traditional unit of measure, equal to one-twentieth of a ton: that’s 100 pounds Avoirdupois in the US, or 112 pounds in the UK (also called an “Imperial hundredweight”).

Since Haddock is British, we can assume he was using the Imperial hundredweight, which would mean the treasure weighed about 450 pounds. If it were only bars of gold, at $1200/ounce, this would be a current value of about $8-9 million, which would need to be adjusted for older times, but would still be a king’s ransom.

Gold coins are worth a bit more than bar gold, as they include the cost of minting.

However, Captain Haddock describes the trove as being composed of “gold, jewels and treasure.” It’s generally the case that precious gems are worth far more than gold, per unit of weight, and jewels (gems fitted into precious metal) enhance their value even more. And “treasure” could refer to works of art or spices such as peppercorns, that were historically worth far more than their weight in gold.

Since The Unicorn was under sail with rum and tobacco, it’s reasonable that they might have been sailing from the New World, and could even have been hauling emeralds from Central and South America.

So a basic valuation in gold would be the bare minimum, and the actual value of the ship’s trove could be considerably more (even many multiples more) than that.

If it was an actual 400 troy oz then today January 14, 2014 with gold at 1,269 USD/oz it would be worth 507,600 USD.

Based on the time period of the stories the movie is based on (1941-44) when gold was 33.85 USD/oz then it would be worth 13,540 USD or adjusted for inflation 179,218.06 USD

I could be wrong, but they might be using the old yet of weight not as "weight" but as a fother, or cart-load. This is also described as six stacks, where each stack is five fortmal, or as a volume of forty bushels; roughly 320 gallons. If we go by the volume measurement, that's roughly 128,000 gallons of gold and jewels, which is very hard to get a price estimate on.

I think, logically in the context of the movie, it would refer to the cartload measure. Providing that kandlin's math is accurate :-), 128,000 gallons would equate to 17,111 cubic feet of gold. Now the weight that many cubic feet of gold comes up to 10,300 TONS!!

Now, of course this is assuming weight of solid gold... not coins or jewelry or gems. But even if we cut that weight in half, I would highly doubt that an 17th century Man O'War could carry 5,000 tons.

Tintin answer, About $1,000,000,000 USD ballpark range.