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.