Economic issues in MMOs and virtual worlds get a ton of attention, but less attention is paid to the 'single player economies.' Richard Knight argues they're no less important — while a bad economic setup won't ruin an otherwise good game, it can put a damper on the proceedings. I can certainly think of a few games that had economic setups that were mind-bogglingly bad in a number of respects:
Games that don't recognise that key fact have what we call a "false depth economy"; a situation in which currency is rapidly devalued until it becomes irrelevant - or possibly even aggravating - to gameplay. Remember putting rupees back into chests in The Legend of Zelda: Twilight Princess? Or ignoring the store in Star Fox Adventures entirely after the first few hours? These are both bright examples of what happens when the economy doesn't work as planned.
I can't argue that good games haven't fallen to this problem before but succeeded nonetheless. Nobody is going to throw down Twilight Princess in disgust over rupee returns. But I can assure you that it's a design problem that every designer wishes they could take back and fix, and one that every player will write down as a negative about their overall experience with a game.
He lays out three principles that ought to be followed — it's a quick article and worth a read if you're interested in game design.