The party is free to all AP holders. You just get free drinks and carni games when you either pay the extra 15 bucks, or if you have the premiere pass. I would not go JUST for the party, stick to your plan.
This part is
free to Preferred AP and Premier AP (on Sept. 28 and 29 from 5-7 PM in the Amity area:
Early park admission to USF for
HHN at 5:00 PM
Early access to one pre-determined haunted house
Preferred seating at the first B&T show
Food and Drink specials
Music and Entertainment
You can upgrade to the VIP Party Pass.
This is free for Premier Passholders, $15.95 for Preferred, and $19.95 for Power Pass (limit 6 tickets per passholder)
In addition to the benefits listed above, passholder who upgrade receive:
Priority entrance into Haunted Houses located near the party zone
Photo Opportunities with select Scareactors
1 free carnival midway game
2 free drinks with souvenir cup (limit one per passholder) of beer, wine, water, or soda followed by Happy Hour drink specials
Restrictions
Must have a paid HHN event ticket for early admission and to enter HHN passholder Party
Price of HHN event ticket includes HHN Passholder party
Advance reservations are highly recommended due to limited space
Upgrade tickets will be available the night of at Guest Services
Passholders can purchase up to 6 upgrade party tickets.
Sales call 1-866-PASS-4-FUN
Only