I'm not sure it's possible to justify it based on the value of the individual items, but I think that the experience can be worth it. I used to think that the dessert parties were too expensive, especially since we traveled as a family of 5. Last summer my husband, my son (the last still at home), and I went to
Disneyland and did the dessert party for World of Color. I primarily booked it because we would have seating and wouldn't have to stand to find a place for a long time. But it was our last night there, and it was just so special to be in that place having that experience with two of my favorite people. Even my husband who finds it more difficult to justify that kind of spending, told me that it was worth it.
My daughter (who now works/lives about 4 hours away) proposed a mother-daughter trip for the week after I drop my "baby" off to college. She didn't get to experience Disneyland with us (and I thought about that all the time we were out there), so I wanted to make this trip extra special. IllumiNations has always been her favorite. I was watching and waiting for the Sparkling Dessert Party to open for our trip and was so sad that it didn't seem like it was going to. And then this party opened! It's going to be our last night just like the World of Color one was. I know that it's going to be special for both of us.
TL;DR - You're Frozen-obsessed daughter will be only this age now. That doesn't mean that you won't still have many years of great trip and great memories, but none will ever be the same as this point in time. If it doesn't break the budget, go for it!