Gold/silver coins where pretty pure, at least in Greek/Roman times. Like 90%-95%.

One approach is to value all currency by weight. And have a coinweight, 1/100th of a pound, be a unit. Then different countries currencies can still exist.

Gold/Plat has about twice the density of Silver/Copper.

Using 500 g is 1 lbs (because lbs suck)

Based off density, this makes "coinweight" SP and CP be about nickle sized and GP/PP be about penny-sized.

(2.5 g for a penny, 5 g nickle, 6.3 g loonie, 7.5 g euro approx)

A 1 euro coin has the weight of 3 pennies. A Canadian "loonie" has the weight of 2.5 pennies.

(5 g for a penny, 10 g nickle, 12.5 g loonie, 15 g euro approx)

(The above assumes the same density)

Per pound:

CP/SP: 100 standard nickle, 200 pennies, 80 loonies, 66 euro-sized.

PP/GP: 100 standard penny, 50 nickles, 40 loonies, 33 euro-sized.

Using the 10 sp per gp, and 100 cp per sp means that prices in cp remain unchanged. Price in gp becomes price in sp, and price in sp is divided by 10.

In an existing module, you can replace any "100 gp" with "1 pound of silver coins". For treasure measured in sp, divide by 10 first.

The nice thing about this is that (a) it makes clear how heavy the coins weigh, by putting that **first**, and (b) it makes different currencies in your world, without making it awkward, because treasure is measured in pounds of specie.