So lets' have a little fun with math shall we. Assuming, as per this discussion, there are 25,000 tickets available for each of the parties and there are 29 party nights, that's 725,000 tickets. Even though they won't all sell out let's say that they do. We'll balance that claim by going with only the cheapest price ticket to each and every night although prices many nights are higher. For simplicity, let's also say that the attendance will be split 50/50 adult/child (remember we're not accounting for higher ticket prices on some nights). So, that's 12,500 tickets at $67 and 12,500 tickets at $72. Plug these numbers in for those 29 parties and for the 2016 MNSSHP season you have a fairly conservative estimate of $50,387,500. Following the same process for the 21 MVMCP at 25,000 tickets per night split 50/50 adult/child, at lowest prices of $81 and $86, you get $43,837,500. I realize there are slightly lower priced DVC and AP tickets for some nights.
So the ever expanding "party season" at WDW is now a roughly estimated $94 million dollar income stream. This does not include parking, food, merchandise, add-on dessert parties, photos, etc. Remember as well that several hours of the party would have most likely been open to existing day ticket holders, so some labor and other expenses would have already been incurred.
Take these numbers loosely, and you can still see why these and other after hours events have become so popularized by WDW. Easter Eggstravaganza at AK anyone? 4th of July Founders Day Bash at EPCOT?