We did MNSSHP last 10/03 and had a great time. Yes, it is extra $$ but it was well worth it! Atmosphere was great, candy for the kids, and best of all seeing MK with all the Halloween themed decorations and riding the rides with little or no wait. We'd do it again, if we're there while its going on.
But seriously though, they give trick-or-treat bags to everyone, even "big kids". The cost is nothing compared to how great the party is. From what I've read on here, it seems the tickets will go on sale at the end of this month or early May.