*Cliche "I'm a leecher" comment goes here*
Okay, so I was in Ulduar the other day and a Disc Priest jokingly cast Power Infusion on me. I noticed my Death Coil had 8 RP removed from its cost (-20% cost, just like the -mana cost of the spell).
Like any responsible adult I quickly ran a search for -mana cost items.
It lead me to Burst of Knowledge, the classic "I wish I had a real healing trinket" item my priest used 3 years ago.
Aaaanyway, here's a screenshot of the result:
http://i31.tinypic.com/4ig9ox.jpg
-100 Mana Cost translates to -10 RP cost.
I assume this stacks with PI and would lead to either 14RP Frost Strikes or some other number because -mana cost isn't that simple.
There is also another trinket, Spark of Hope, that is a passive -42 mana cost that I assume would be -4RP. Currently I have healers in front of me on that one so I can't say I can test that one.