I actually submitted a monetary system table to CRAWL! that randomizes the exchange rates. Here it is:
Money is merely an easy medium of exchange, and a way to keep that exchange portable. Most peasants, of course, exist solely in a barter economy, and as such are not very mobile. Adventurers are different, and currency in general (and coins in specific) are valuable because they are light and portable and can be readily exchanged for necessities: food, shelter, repairs, upgrades to gear, comfort, or placating an angry deity at the local shrine.
But coins are made of metal that can be dug up out of the ground and minted by the enterprising, and so there are, and should be, fluctuations in exchange rates. Here's a quick table to introduce a little randomness into the exchange rates of the various monetary metals, as well as a few possibilities for other, nonstandard coins and suggested exchange hierarchies.
1 copper piece (cp)
1 silver piece (sp) = 8 + 1d14 cp
1 gold piece (gp) = 4 + 1d16 sp
1 platinum piece (pp) = 8 + 1d24 gp
1 electrum piece (ep) (alloy of gold and silver) = 2 + d3 silver pieces
1 bronze piece (bp) (alloy of tin and copper) = 3 + d5 copper pieces
1 iron piece (ip) = 7 + d3 cp (thus, could possibly roughly equate with sp)
1 steel piece (*p) = 20 + d30 ip (highly unlikely to be used as coinage, unless silver / gold are very, very rare)
You can adjust these values according to your own campaign, of course, or you can roll them up fresh each time the adventurers walk into a new town. (Don't forget the local Baron's tax!)
Does this make things more complicated than 10:1 ratios? Of course, but it adds variability and realism, and, as always, you don't have to strictly adhere to it. Suppose you rolled that 9 cp = 1 sp, and 17 sp = 1 gp. Just because your PC wants to trade 304 sp for some gp doesn't mean you need to break out the calculator and make exact change for them... round it off at 15, or less, if the PC is pushy or has a bad personality modifier.
Hamun Ry (Wiz 4)
Str 10 Agi 15 (+1) Sta 11 Per 11 Int 17 (+2) Luc 10 (Unholy House). Align: C. AC: 14. HP: 13. Melee +1, Ranged +2. Crit: d8, I. Save: Ref +5, Fort +3, Will +4.
Spells: 1: Choking Cloud, Comprehend Languages, Detect Magic (odd crystal growths), Magic Missile (mirror images), Runic Alphabet (Mortal) (ravenously hungry), Ventriloquism (rain of frogs)
2: Detect Invisible, Levitate (extremely difficult, d14), Mirror Image (20% chance to raise/lower luck by 1d3 points).
Equip: Ring of the Sand Djinn: +2 AC/saves, Invisibility for 1min/spellburn point, or unmake for great, unknown effect. Padded Armor, Longsword, Longbow, quiver w/20 steel-tipped arrows, 10 silver-tipped arrows, backpack, spellbook, quill and ink, sturdy parchment (10 sheets), 5 days rations, high leather boots, belt w/ belt pouch, gray robe, dark gray hooded cloak. Also carries 3 small mechanical toys: wind-up mouse, wooden puzzle cube, small jewelry box that plays a little tune.