5 friends-Albert, Bob, Casey, Dexter, and Edward are all going to the movies. Because of the difference of age, all of them have different ticket prices. Casey’s ticket price is also the median of the five friends’ ticket prices. Casey, Dexter, and Edward’s ticket prices are in an arithmetic sequence, respectively. Albert, Bob and Casey’s ticket prices are also in an arithmetic sequence, respectively. If Bob’s ticket costs three dollars less than Casey’s ticket, and the mean of the 5 ticket prices is the mean of Casey and Dexter’s ticket prices, then find the positive difference between Casey and Dexter's ticket prices.