The money system used in the campaign is generic and decimal. One copper penny (cp) is the smallest useful unit of currency; there are ten copper pennies to the silver penny (sp) and ten silver pennies to the gold piece (gp). A hundred gold pieces make one Mark, which is a whole lot of cash. A small loaf of bread or a mug of beer will cost you a couple of pennies, a longsword of moderate quality will cost about 25 shillings (sp).
By an amazing stroke of coincidence, all coins (copper, silver and gold) weigh about 10 grams (100 to the kilogram). A copper piece is about 20mm in diameter, a silver piece about 16mm, and a gold piece 10mm.
As far as the game goes, the most common unit of exchange is the shilling (sp) and almost all prices are quoted in sp.
Copper pennies (cp), although actually more commonly seen in the markets than silver, are just too cumbersome for most adventurers to bother with. Besides, how would it look to be seen paying with copper? It's just not done. People would talk.
Gold coinage is relatively rare — think how infrequently you see anyone paying for a meat pie with a hundred dollar note. Of course your aristocrats, big-noters and other social climbers will want to flash gold about, but letting the hoi-polloi think that you've got a purse full of gold is a guaranteed way of making yourself a target for thievery.