Find, post, and buy the "Collect Ghost Studs" extra. The parcel was either in Harry's bedroom or on the courtyard fountain (You must 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, resulting in easy money. A full trip from the Dorm to the Great Hall is worth about 100,000.