For myself, I would factor all recipe and salvage drops at vendor prices as they are guaranteed, whereas the AH prices fluctuate. It would most likely give a lower standard but offer the option of 'you can maximize profits by marketeering'.
I usually vendor white drops and AH yellow, orange and purple. But to try to nail an average to that, well I guess I dont pay enough attention or keep good records. Drops are too random and AH prices are too flexible to really depend on (for my own extremely general calculations). I do not craft or convert so I save that amount of time for more face punching.
Math and I dont get along, so that's also a factor.
I am so glad this discussion has finally returned some useful insight and has stopped being a mud throwing contest. Thank you all for that 😀