Who knows Dark Magic?
Solution 1:
If the DS version is the same in this regard, it would be Bellatrix (see this question for location).
Solution 2:
On the DS version if thats what you're playing I would go for Alecto Carrow on price. You can also get Dolores Umbridge for the same reason she is cheap. But if you do have a lot of money to spend its Tom Riddle or Bellatrix LeStrange. Just a note.
- Alecto Carrow: $20,000
- Dolores Umbridge: $50,000
- Tom Riddle: $500,000
- Bellatrix LeStrange: $500,000