Compute the mean price of procedures 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"
mean price refers to AVG(T3.cost); procedures refers to T3, procedures John Wen was trained in involves JOIN between tables physician, trained_in, and procedures; John Wen refers to T1.name = 'John Wen';