Extra days on a multi-day ticket cost about $20 each after you buy 4 days. A party ticket is flat-out about $100.
And I speak from experience when I say that 4 party nights a week ruins everyone’s trip that week. Aside from what it does to the ability to enjoy the MK, Epcot is slammed with people going there instead. Try as they might, AK is not a night time park. Maybe when the Studios construction is finished, it will help, but this year will be horrible during party season.
The only way parties can be worked around has nothing to do with planning. It is all about how much more money you’re willing and able to spend. Want to see HEA during party season without being stuck in a sea of humanity? That will cost you $84 plus tax and admission for a dessert party. Want to stay in the MK late one night? That’ll be $100 each. Oh wait, you can just hop to another park for another $75 on top of your ticket.
It makes me sad that Disney is actively trying to price out entire segments of their market.