Find, post and purchase the "Collect Ghost Studs" extra. The parcel was either in Harry's bedroom or on the courtyard fountain (You will need to have learned the Leviosa spell first). Once activated the Ghost Studs are worth 1,000 each. "Get lost" the ghost will return to you and lay more studs -- it's easy money. A full trip from the Dorm to the Great Hall is worth about 100,000.