Copper pennies weigh 3.11 grams. Copper is currently US$10987 per tonne https://www.metal.com/en/prices/LME_CA_3M so a copper penny is worth 3.4¢. This is a surprisingly low number to me; I would have expected it to be closer to 10¢ or 20¢, since presumably it was about 1¢ of copper when it was still copper.
By comparison, a silver dime (90% silver, 10% copper) is 2.268 grams, and silver is US$1486.77/kg https://www.metal.com/en/prices/201102250392, so the dime contains about US$3.03 worth of silver. From the point of view of an 18th- or 19th-century person, for whom the purpose of the mint was to certify the value of the precious metal in the coin by stamping it, the dollar has lost 29/30 (97%) of its value since minting of silver coins ended.