I don't mean to take this thread too far off topic, but....
You can hold private events at all the same places in Epcot that weddings & conventions do, but there are minimum numbers of people required at each one and a per-person food and beverage minimum, so the availability and cost will depend on the size of your group.
Because our party was just the 2 of us, we had it at UK Pubside, which is a tiny patio between UK Lower and the patio of the Rose & Crown.
The food & beverage minimum at UK Pubside is only $220 total (so I guess that's Epcot's $22/person food & beverage minimum x 10-person minimum). When you add in the $100 venue-rental fee, it's $320 plus tax and service charge.
If you wanted to do, say, UK Lochside, you would need at least 20 people or have to pay the food and beverage minimum of $22/person for 20 people even if you didn't have that many. Then add in the venue-rental fee of $325 and a setup fee of $75 and that'll give you an approximate idea.
Info on the venue-rental fees and minimum # of guests can be found in the various dessert party threads posted on this board over the last few months....