Keep in mind that most professors don't require you to purchase anything in order to participate in classes. However, here are some ways you can earn money.
- Purchase lottery tickets through the bank.
- If you average 5 posts a day, you are eligible for Welfare and can apply through the bank.
- Some clubs at Hogwarts might offer money for completing activities.
- Usually when you're a 2nd-year or higher, you can be a TA, club moderator, or member of the JW. These positions sometimes offer money.
- Hogwarts has a student jobs thread that is updated throughout the term where professors will post small jobs that students can do for money.