The question apart from medical point of view, is attached with humanity and sentimentality. Firstly, we have to see for whom the kidney is required i.e. his/her age, family background etc. From medical point of view, one kidney out of two kidneys of each human being can be donated but both the donor and the receiver are to pass rest of their lives under various restrictions. Yea, if it is required to save the life of a 18 year old guy, there is justification in donating kidney. Though taking money against kidney donation is not ethically supported,the cost of medical expenses the donor has to bear for rest of life requires monetary support.