How to Calculate Agile Project Budgets ?

According to a survey conducted by the Harvard Business Review, 17% of IT projects exceed costs by 200%. This is due to the desire to make an exact estimate for the entire project before starting work. ANY estimation or budget given before the actual work is done, is always an educated “guesstimate”. The fault tolerance of the budget depends on the experience and the risk associated with the particular project at hand.  “Budgeting agile projects is impossible because of the nature of agile projects”. so how should we do it?