I always make sure that the user/client/business sponsor creates the "cost of downtime" figure. I take a role of advisor and facilitator in determining the cost of down time. This does several things -- If users determine the cost of downtime thenuptime must have value (IT has value), and second if downtime has a business cost obviously avoiding downtime might be worthy of an additional dollars (and the business justifies additional spending - not IT).

Never miss a chance to get the business users involved in IT decisions.