Unfortunately, a player-based economy in Minecraft just doesn't work. I'm not sure it's possible without completely different gameplay based entirely on it. People go at different paces, so the value of an item can't stabilize.
Imagine a small server with just a few people. You're all friends and you set up these "prices" in a meeting early on that you all agree to. Let's say that you can buy 5 iron ingots with 20 wheat, for example. Well, that's all well and good in the very beginning, but people tend to go at their own pace. Iron ingots, for example, may be pretty useful to buy early on, and maybe that supplier needs some wheat because he skipped his farm and started mining right off. Great! But the next day, everyone's got a mine and a farm and the value for both items drops to nothing.
In comes a new player who has nothing. He or she is still looking for iron and wheat, but at this point, both are trivial to everyone else. Well, you don't need that player's wheat. Just toss them a few trivial stacks of iron if they need it.
Ultimately, this game design simply can't sustain an economy without a complete redesign. Sprout gets away with some in-game economy because it has scavenger mode and using in-game currency might be the only way to make your armor. Without some kind of limitation, like, say, each player gets a "job" so only one guy can make wheat and someone else has all the iron or whatever, the value for everything will drop to zero very quickly.