Anyone can buy HHN tickets. Generally, they are available on the website or at the gate. Ticket prices are about:
For AP holders:
Peak nights (Friday, Saturday): $30
Off-peak nights: $20
Stay and Scream tickets (buy a day ticket to the park and do HHN the same night) run a bit more than the AP holder rates.
If you just want to do HHN and have no AP or same day ticket, then the price is about the same as a day ticket: around $50.