Quite a few items should not be disenchanted.

My Pally/DK team dropped around 20K on the Conquerors Plate set, which is level 72 greens.
It was an average of 500g per piece.
They were green drops, off of random level appropriate mobs... meaning that farming the set might have required 200,000 kills of level 70-74 mobs.
But the AH is a good source for those.

You can download the Undermine Journal addon.
An account (free register on their site) lets you download the addon for four servers/factions per day.
I've done Cenarius Ally, Cenarius Horde, Magtheridon Ally and Magtheridon Horde, for my four (as those are the two servers I play on, and something might be worth a lot more on the other faction).
The addon gives you current and historical AH pricing on almost everything.