Thanks to everyone - I now have, roughly, a currency system that has some coherence. No doubt most of the metals are far too expensive, but since I doubt I'm ever going to have to go into the comparative costs of copper and tin to an ounce of silver, I think I'll be alright.
My coins are bronze, silver and gold, where gold is used only for really expensive items - like fine armour and warhorses and bejewelled items and exquisite silk, lace and velvet gowns with seed pearls and gold thread and crimson dye and whathaveyou - and storing away your wealth. They're big coins - one ounce in weight - too.
And yes, this is sad, but I actually weighed some coins. Semi-useless facts discovered? A British fifty-pence piece weighs almost exactly a quarter-ounce, while a pound coin weighs between a half-ounce and a quarter-ounce (my mum uses imperial weights for cooking and I was brought up on the same ones - at least when it comes to cooking - contrary to popular belief, the imperial weights and measurements are still used quite frequently in Britain - everything's in miles per hour because it would've been too expensive to change all the roadsigns to kmph and distances in km).
So anyway, I have three bronze coins, two silver coins and a gold coin. The smallest bronze coin is used for inexpensive stuff, like a loaf of bread, half a dozen eggs, or a couple of pints of ale. The next is the general day-to-day coin for buying food or paying for a night's meal and lodgings at an inn, while several can purchase a pair of boots and five will get you a stone of wrought iron to be worked. The next is for larger stuff - worth ten of the second coin - it works for clothes and such. Then two silver coins, one worth double the other, for expensive items, including rent, weapons and the expensive cloths and such of merchants and nobility. Basically:
Bronze coin 1
Bronze coin 2 = 2 BC1
Bronze coin 3 = 10 BC2, 20 BC1
Silver coin 1 = 10 BC3, 100 BC2
Silver coin 2 = 2 SC1
Gold = 80 SC2 (gold being equal to 20 ounces of silver, and Silver coin 2 being a quarter-ounce in weight)
The weights don't perfectly work out according to value - I think one might be worth more than its metal which makes it perfect for forgery - no system's perfect, and even I have limits to my pedantry. As long as the rest of the system works, I'll cope.
So far I haven't calculated anyone's wages, but that's not too important for the current novel and I now have reference lists to work off to create a reasonable comparison (an imperial penny during the medieval period is equal to the second bronze coin).
So, all I need to do now is name the damn things! And - as my dad pointed out because I getting him to tell me shillings to a pound and stuff because he *remembers* using all those old coins - then I have to create the nicknames (you know, a dollar being a buck, a pound sterling being a quid, a shilling being a bob). I'm thinking about something Welsh as the official names, and then references to, maybe, the symbol stamped on the back of the coin for nickname sources. And rather than the monarch's head, the coins simply hold a version of the royal symbol on the front, with the year of minting.
Any suggestions, let me know!