The Romans only imported what they couldn't steal. The drain on their precious metals came from the need to pay vast armies, so they diluted their coinage. The Roman Emperors devised many ingenious methods to debase and make their money cheaper. Silver coins went through a series of devaluation stages - reducing the size of the coins, shrinking the silver content while increasing the alloys etc. It got so bad, that by the year 225 bronze coins were merely washed with a scant coating of silver. This was done for one simple reason - to have enough coinage to distribute to their vast mercenary armies. Making it worse was the populace, who had a bad habit of scrapping off a small amount of any coin that came their way. By the time of the late empire, their currency was worthless and their armies revolted as a result, returning to their barbarian tribes so that they could scrape off a chunk off the empire instead.

If one is looking for a model of the fall of the American Empire, the model would be Britain, not Rome.