I'm currently experimenting with coins.
TL;DR version: Bronze Farthing, Copper Penny, Silver Star, Gold Orb, and Mithril Mark.
Currently set at ratios of 4:1, 5:1, 10:1, and then 10:1. That is 1 mm = 10 go = 100 ss = 500 cp = 2,000 bf.
Long version:
For currency, I've done a radical overhaul, building the equipment lists from the ground up. I'm on my second major revision (and I don't know how many minor ones), trying to strike a compromise between game play, vaguely historical/fantastical sounding units, and a focus on main late dark age, early middle ages technology.
Complicating it is that I've deliberately left in some late middle ages options under the guise of a previous golden age. So, for example, a few people know how to make plate armor, but not so many that the weapons list has completely gone into late medieval territory. Yeah, I know, some of this is mere rationalization for a hodge-podge of disparate stuff in one game.
I also wanted a silver standard, that was easy to convert from many existing fantasy games and from historical sources. The nod to late dark ages, early medieval is that weapons and clothes are generally much more expensive, and the higher tech weapons and armor are really expensive. This gives characters something to spend money on before they start finding magic (relatively late in the power levels).
Throw all of that in a blender and do some judicious rounding, and I got those coins.
Seriously considering bumping that last gold/mithril ratio even higher into 20:1, but the "mark" part is about right given the expected purity, size, and ratio of silver and gold (devaluing gold to the lower end of its range, but that's OK given the mixed tech setting). Even though "mark" isn't a real coin but an accounting measure. Of course, mithril isn't real either, so maybe that cancels out.
Delta's Hotspot musings on currency helped a lot with this last pass, in particular setting the silver standard at about the level of the "Groat", or about 4 silver pennies. I didn't use "Groat" because it's a much later coin, but maybe that's me being too picky. OTOH, after a lot of back and forth with various real-world silver and gold names, I didn't find any that didn't invoke either a much later period or imply ratios that are not at all a match for the above.
To reconcile it for game play, the above ratios assume all of these coins are about American quarter size (though a little thicker), with the silver and gold relatively pure, the copper a thin coating on a silver shell, and the bronze a relatively impure copper/bronze mix (that is, lots of impurities that aren't valuable). In reality, copper and bronze aren't used at the same time, and would be close to equal in value. That gives me about 80 silver per pound bar. Which still isn't technically correct, but is in the ballpark for real-world silver. It means that I can use Delta's neat trick of of reading historical equipment list "pennies" (d) as "cp" even with copper penny, "shillings" as x3 silver, and "pounds" as x6 gold. I can also convert most early D&D adventures by dividing treasure values by 1/10th. This makes copper a little more worth hauling out.
The last thing that prompted this particular revision is that I had a slightly easier to convert system before, but the scale and naming was off. I had two different silver coins and a lot of overlap in the sounds, and it was causing all kinds of issues in play. You'll note that a player can use shorthand of "bronze, copper, silver, gold, mithril" or use the coin names, even mixing those up, without confusing the issue. Standard sizes also help. I really wanted to use the dark age "silver penny" as the base, but it just doesn't work with all the other considerations above. I also like the alternate names of "silver moon" and "golden sun", but that provides more overlap and it inconveniently makes the gold abbreviation as "gs" which has a real-world reference inconsistent with the its value.
Designing this system caused me multiple headaches. I hope it works, because I don't want to redo it.