How much is a gold lire coin worth? 6.45g and 0.900/1000.?

If it weighs 6.45g then it's a 20 lire gold coin from Italy under the reign of Vittorio Emanuele II. But V.Emanuele II reigned from 1861-1878, so it's not possible for your gold coin to be dated 1860. Perhaps you just read the date wrongly, or the coin is a replica. In general, V.Emanuele II's 20 lires of the 1860s period are valued at about $125 each in average circulated condition to about $150 for nicer circulated ones. The coin has a 0.900 gold fineness and contains 0.1867 oz of pure gold each if unused.