If you want to continue down mister Chris's thought process of valuing resources based on relative abundance, you can use the fact that fortune III on average increases drops for diamond by 120% to set a lower limit on the value. If you assume all diamonds are mined with fortune III, you get 11 iron per diamond (this is the lowest reasonable ratio). You would need more information about how frequently different pickaxes are used, but, to set limits, in terms of relative abundance, there are 11-25 iron per diamond.

But, this is still oversimplified. A majority of the mining being done on the server occurs at Y =< 16, where all diamond ore is generated. This is what BluePlauge is referring to. If we assume iron veins are uniformly distributed in 0 < Y < 64, the relative amount of iron available to players mining at "diamond level" is 1/4 the absolute amount. So, for these players, the lower limit is 3 iron per diamond. But now to determine to actual value, you have to consider another variable: how often people mine in this fashion.

In the end though, I think this is a pointless exercise. As Sofatroll said, iron can be easily farmed. Its a commodity not a currency. I wouldn't value the US dollar on the number of potatoes in supply. And given though people only ever purchase with diamonds, and rarely with iron, I don't feel especially to determine an exchange rate.