Hi Tara! For me, it is absolutely worth the cost of attending
Mickey's Not-So-Scary Halloween Party! I get so excited when the dates are announced as this is definitely on my must-do list each year. Some of the things I enjoy at the party are the extra character meet and greets, Hallo-Wishes, the parade and of course trick or treating in Magic Kingdom! I also find it fun to dress in costume which really gets you in the spirit of the party!
However, you are right - it can be expensive, especially when you buy multiple tickets for your family. I typically do not go to a park the day of a party and instead sleep in, enjoy swimming at the resort or another recreational activity so I justify the ticket as my going to the park ticket. I realize that it doesn't save a lot of money if you have a multi-day ticket but for me, it's a way to stretch my ticket use. Also, if you buy early, you can usually save a few dollars on each ticket. Disney also offers discounts to Annual Passholders and DVC members. So keep an eye out for any discounts that may apply to you.
If you decide to go, have a spooktacular time!