MNSSHP is totally worth it!! It is extra, but maybe you can use your GC? we have gone twice and loved it. Both times the park was pretty empty, the firework show is amazing, and its great to see all the characters dressed up. We were able to ride splash mountain 2-3 times consecutively because there weren't many people in line. It was great to not get out and have to get in line again. Also the haunted mansion looks amazing as the nightmare before Christmas!
MNSSHP is totally worth it! It does cost extra, but it's neat to be in the park when there's not a big crowd. The Halloween parade and fireworks are really neat; something you don't see during a regular day visit.