Al's latest report on MiceAge states the estimated cost of the early closing for the Tourism private party somewhere... You'd have to cover the estimated takings for the day at least before they'd entertain your presence.
Private Party prices vary greatly depending on what you want to have included. There are so many variables and options, that it's almost true that no two Private Parties cost the same.
The only time I recall seeing a set price for a Disneyland Private Party was for a number of years when it was in the Neiman-Marcus Christmas Catalog. It was always a full page ad with a picture of the castle at night...and it said something like, "An intimate Private Party at Disneyland for you and 35,000 of your closest friends. $500,000" That was like in the 1980's though.
Don't know what type of entertainment or food was included in that price. I never called because I just couldn't come up with any more than 23,000 intimate friends.
You can also just rent out as little as one land. You get that land and its attractions, some entertainment, and a dinner with beer or wine. They are/were called "Enchanted Evenings."
But really...you can customize your Private Party to be just one or two lands...to the hold ball of wax...and everything inbetween.
Even what's called Mix-ins...which aren't very private at all.