Okay so before anyone says it, I know IC coins already exist. This is not my suggestion.
First off i believe that IC coins are pretty easy to create and altho receiving them gives you valuable iron ore, I have found myself with too much iron in the past. My suggestion is to add 3 different coins that have separate values. Something like Copper, Silver, Gold. Copper is a very common ore to find, therefore a copper coin would not amount to much value. But things like bread could be sold for 5 copper or something. Silver, an ore that would need to be created, would be less common, approximately double the commoness (new word =P) of gold, and about half of copper. Silver would be found anywhere from bedrock to (16 layers above gold). Silver being more rare would be valued higher then copper (also silver would need created uses to actually give it a use when people find it. Useability = Value). Gold being the most rare of the 3 would be as it is now, found deep and not super commonly. Most valuable metal (maybe create more uses for gold too). This would add a better rp ability in SMP, obviously pretty useless in SSP. Just a thought, tell me what you think.