I can easily imagine a high end catered party costing that much.

I don't think there is an upper limit on how much hosting a party costs. You can always go fancier if you have money to burn.

Christmas dinner for my immediate family almost $500. That was pretty much dinner and our favorite appetizers but does not count the liquor and wine. This was just me, my ex, five kids, a daughter in law and grandchild. I can see getting to 3k pretty fast.

Well 50 people attended. So. Yea? Two parents for 21 students or so plus a few extras.

That honestly seems quite cheap for 'very upper class' where I imagine everyone's suited and booted, dressed up for the evening, possibly some live music, etc.