Because it doesn't work in reverse. What most of us would call world coinage means non US coinage. And the reason that non US coinage is much less expensive than US coinage is because there is less demand for world coinage. But in recent years that has changed somewhat as world coinage has become much more popular among US collectors. As a result values for world coinage have risen significantly and I expect them to continue to rise for some to come.
Only time I ever got lucky with foreign coins was with British stuff, I wasn't even trying... Was at an antique store in Georgia, the lady had a basket full of British 1 pound and 2 pound coins.. $1 each or 3 for $2. If memory serves I spent $20 to buy 30 pounds ($45 worth). At the time I was travelling to the UK almost weekly on business Still, whenever I have seen US coins for sale in Britain or elsewhere in Europe they are highly priced.. grr