What is the average cost of procedures that physician John Wen was trained in?
SELECT avg(T3.cost) FROM physician AS T1 JOIN trained_in AS T2 ON T1.employeeid  =  T2.physician JOIN procedures AS T3 ON T3.code  =  T2.treatment WHERE T1.name  =  "John Wen"
average cost = AVG(procedure_cost); procedures that physician John Wen was trained in involves joining tables 'physician', 'trained_in', and 'procedures'; John Wen's procedures are identified by matching physician name in the 'physician' table and linking through 'trained_in' to 'procedures'.