There is no concept in World of Warcraft for how to measure what level of profit you should make to cover the risk you are taking. For the record, in the real world it is about 2% (10 year US Treasury yields).
However, in World of Warcraft i am able to determine what level of profit you should make for each hour of time spent.
i do this with reference to gathering herbs. Though it is not without its flaws.
I can gather 20 stacks of whiptail in an hour. I can sell those at present for 40 gold a stack (2 gold a herb) over a 24 hour posting on the auction house. Therefore, for each hour of gathering i can earn 800 gold.
Therefore, i determine that my gold making activities must make more than 800 gold per hour else i am better off gathering.
Following this through a bit more. At present, the supply of herbs is low, hence i believe i could gather for 2 hours a day and therefore sell 40 stacks of whiptail each day at 40 gold a stack for 7 days (i.e. 2 hours gathering every day).
That gives me 11200 gold per week.
Therefore, i determine that if i spend 2 hours a day on gold making activities then i must make over 11200 gold per week – else i should revert to gathering.
Though markets are slow, my Inscription profession is earning me over 11200 gold per week at present before i add in the profits from Enchanting and flipping.