The best planned gaming economy tanks when the masses get to it. The fastest and best thought out reactions win the day. I have seen some amazingly well planned out games go full toilet flush in just the first week, people will exploit the darndest things, and stuff you never never planned to hold value will, and the things you thought will be hoarded will be blown off entirely. A seriously rigorous beta can help but it also works in favor of those looking to find and profit from a exploit. Keeping a watchful eye on economic trends closer to launch can also help, but again... the masses do the darndest things, and when you say that the game is live and no more wipes is when people go full throttle on those exploits.
A good plan is a great start, getting ready to tackle the issues that arise is where you will make or break the game!
