Devaluation happened primarily during the middle ages in Britain, beginning mostly with William the Conquerer and going from there, where coin standards were constantly being shifted (how much silver is in a silver coin, how much gold...). However, until Henry the VIII coins were worth purely their weight, after Henry many coins were valued based on an arbitrary standard (X pennies to a Y) instead of their weight.
In RPG terms I always, ALWAYS, use coinage valued by its weight, not an arbitrary numbering system. If 100 silver coins are the same value as a gold coin, then that much raw silver is worth that much raw gold (Right this second it's closer to 63 to 1 based on USD value of a both). Since coinage is never a set weight from mint to mint, let alone era to era, it's much easier to just say treasure is worth weight, not value, and attach the weight to something simple (As an example, 1 ounce of silver could keep a farm running for a month) and extrapolate from there. So a party doesn't find 10,000 silver coins in a pile, they find, say, a hundred ounces of silver, which is enough to fund a barony for quite some time. Or get blown on equipment and whores. Whatever.