This is the good old, CRAFTing; DISENCHANTing.
1. Make sure you have the Pattern: Bolt of Soulcloth - Item - World of Warcraft (Pattern: Bolt of Soulcloth) and the Pattern: Soulcloth Gloves - Item - World of Warcraft (Pattern: Soulcloth Gloves) - Just look at the tab ''Sold by'' in Wowhead to know who sells the patterns.
2. Learn the patterns. (If you haven't already)
3. Buy Soul Essence [50s-85s each], Netherweave Cloth [2.40g-2.75g a stack] and Knothide Leather [50s-1g] each.
4. Makes Bolts of Soulcloth and Bolt of Netherweaves.
5. Go buy Rune Threads. (Vendor)
6. Make the gloves, and disenchant them.
Now, the gloves has a 100% chance to give Void Crystal.
But the best thing is, it has a chance to give 2, which makes double profit.
With patch 2.4.2 price of the Void Crystal has increased to 40g-50g.
I've been doing that for over 1 month, and always made profit.
Enjoy.