Just the other day I scored a literal mother lode of stuff to disenchant. I spent 2300 gold on about 450 items to disenchant. Not all of the items I purchased were Outland greens; figure 375 of them were.
Out of all the items I disenchanted, I received a grand total of 5 Large Prismatic Shards. If there is a 5% chance that a level 64+ green can disenchant into a Large Prismatic, I should have received about 12-14 from the 375 or so 64+ greens I disenchanted. From my example, there was an actual 1.33% chance of getting one.
Another indirect reason why Large Prismatics are climbing in price are more and more fresh 70 alts bypassing standard (and even heroic, sans the daily) 5-man instances and going straight into PvP.
What can be done to alleviate this issue?
Changes that can be done immediately:
1. Increase the chance a level 64+ green can disenchant into a Large Prismatic. A very large percentage of level 64+ green is disenchanted in the first place; if more Large Prismatics come out of your standard issue random green item.
Changes that can be done by the next patch:
2. Decrease the allure of PvP by increasing the amount and quality of reputation rewards (and thus give a casual person a guaranteed reward for time spent, just like PvP does). This will encourage more instance runs, thus generating more Large Prismatic shards from unwanted drops.
3. Increase the quality of crafted epic items that require Primal Nethers. (Again, a casual person can see a crafted epic item as a guaranteed reward for time spent farming mats or cash for them) This will cause more people to farm materials, thus increasing the supply of standard issue random green items, and thus a chance for more LPS.
4. Add more itemized Badge of Justice gear (read: leather with defense rating). This will further encourage people running instances, in this case a Heroic, which will again increase supply of LPS from unwanted drops.