2.4 is introducing a new recipe to shatter a Void Crystal into two Large Prismatic Shards (once every 23 hours).

The current price for Void Crystals is 15-22 gold per piece, and the price for 1 Large Prismatic Shard is 25-30 gold.

http://www.wowdb.com/item.aspx?id=34872

Once 2.4 is on live servers, the price for Void Crystals will surely double, so if you stack them up now you should be able to make a decent profit by selling Void Crystals and Large Prismatic Shards (although Large Prismatic Shard will probably drop in price over time).