Application outsourcing is a major IT business, with an estimated global market size of 49 billion for 2014. In a transition project the involved parties in an application outsourcing contract need to move from a current situation to a target situation. Generally the proprietor asks the IT service provider to provide a (fixed) quotation for the application services including the transition project, without supplying extensive details about the application portfolio. In this paper the authors present a model for cost estimation of transition projects in application outsourcing.