It depends on how fancy of a place you want to get, and if food is included. Last time, in Costa Rica, we had 16 people in a $4 million dollar mansion with a private chef preparing all our meals. It cost about $10,000.
You can find great places on http://homeaway.com/
Here are a couple of examples:
Sleeps 24 - $4,501/week - http://demeure.com/properties/villa-turquesa--2
Sleeps 13 - $4,569/week - http://demeure.com/properties/casa-fermentini