The other day, I tried to shift your attention to cooking recipe's you get from quests and how well they sell.
Well, this time I would like to point out that the same applies to all other rare recipes. You should keep an eye on those. Scan the auction house for them, check every recipe you get as a drop, pretty much everything you cant buy from a vendor.
Most of those things usually sell for a good price on the auction house, especially stuff that cannot be farmed. A good example would be Recipe: Heroic Potion. This one, while somewhat farmable, is extremely rare and sells for several hundred gold each, sometimes even over 1000.
A decent way to get some rare recipes is from Sealed Scroll Case which you can get in Zul'Aman (more info on the wowhead page). Just remember a basic rule, if you have seen it for the first time and you already have a level 80, then it's probably rare.