This is part of Disney's new model of pricing based on projected demand. Much like how they now charge more for buffets during holiday periods, not because they cost more to operate, but because demand greatly exceeds limited seating, and they simply feel like they can get away with it.
To put it simply, they charge more, because they can. It's guest-unfriendly and comes off as tacky and money-grubbing, but they're not concerned about how you feel about the practice, only how much money you can shovel at them.
You don't get any greater experience on the party days that are more expensive; the offerings are still the same. In fact, it's more likely to be a lesser experience, because they park will be quite crowded on those peak-demand nights. Your best bet would be to try to attend on a quieter day midweek. You'll have a better chance at getting spots for the parade, fireworks, and shorter lines for rides or characters. A better experience and less money spent.
Also, you didn't ask, but with a 5 year old, if you're already doing a lot of activities that day, its probably better to go to the party another night anyway. It can be a very long and tiring day to do parks all day, and all night for the party, especially with kids. Since you can go to the party starting at 4pm, lots of folks choose to use the daytime for swimming, relaxing at their resort, or visiting DD, and then head to the party early.
we love the Halloween party. It's actually our favorite.
