To clarify the question about how to establish the value of coins that have gone out of circulation, Rava brings a baraita that discusses the case of pidyon ma’aser sheni. On certain years of the agricultural cycle, a farmer is obligated to bring ma’aser sheni – to tithe his crops and take them to Jerusalem where they must be eaten in holiness within the city. The Torah states clearly (Devarim 14:25) that if someone cannot bring those fruits to Jerusalem because of the distance or because of the sheer volume of fruit, he can exchange them for money – specifically for coins – that must be brought to Jerusalem and exchanged for food that will be eaten in the city. The baraita teaches that the coins used for this purpose must be usable coins, which excludes ma’ot kozbiyot, ma’ot Yerushalmiyot and ma’ot shel melakhim haRishonim.- none of which were used in Jerusalem as legal tender.
While some suggest that ma’ot kozbiyot are coins from a foreign country, most of the commentaries identify them as coins minted by bar Kozibah, more popularly known as Bar Kokhba, leader of the great revolt against Rome. During the short-lived independent Jewish rule in Jerusalem at that time, Bar Kokhba minted coins. As can be imagined, these coins that celebrated independence were forbidden to be used by the Roman government. The ma’ot Yerushalmiyot were also coins minted by Jewish rulers in Israel that were removed from circulation by the Roman government for political reasons. Ma’ot melakhim haRishonim – money minted by the early kings – may simply refer to coins that became old and were replaced by new ones.