I've been a couple of these before ... not the whole park rented out, but portions.
One year, it was the Seas at Epcot - we could wonder through the aquarium where they had food and drink stations set up all over, and there were Turtle Talk with Crush showings (which were fun because it was all adults being silly). Then afterward, they led us to a private area right by the water for desserts and great views of IlluniNations.
Another year, they rented out the Atlantic Dance Hall on the Boardwalk and we had our food and drinks there, while a band played.
Another conference had rented out Islands of Adventure at Universal Studios - we had marvel charcters running around, food, drinks and we could ride the Hulk coaster, the Spiderman ride or the Tower one (can't remember the name).
Have no idea of the costs, but they were all great experiences for the conference attendees!