We don't need a diamond exchange what we need is /fix to be modified to require materials to be used when /fixing something! That will solve some the Diamond/Iron price problems.
~ @Hornemans
OR we could have a small cost when /fixing, like $1/durability point (e.g. fixing a diamond pick with 1 durability left would cost $1560) It would take some money out of the economy (if that's still a concern), lower the value of Elite slightly (since one could argue that it has too many perks), re-value Unbreaking to Elite+ (/fix /fix /fix!), and we could possibly give Supremes and Ultimates discounts/no cost to this, adding another desperately needed perk.