Gold coins, or crowns, are the standard currency on Britannia and Killorn Keep. Smaller coins exist, including silver pieces and copper pence, whose value is explained by Terri in Ultima VI.
These smaller value coins are seen as a trinket in Ultima I, but aren't seen in the games themselves after Ultima I.
It is somewhat difficult to determine the worth of the gold coins, but for a rough estimate, a two-handed sword costs roughly 80-100 gold coins, while beef costs 5-7 gold coins. The Royal Seal is on the front of the Britannian gold coin, while the back side shows the Codex Symbol.