I think the least expensive option is rooms at a value, or perhaps even at a moderate resort. The villas are not cheap, however they do offer a means to have a lot of people stay together. I think that if you choose a villa, and we have done so, you need to be sure the people in each are used to traveling together.
My family travels as extended family often and we are able to share space, but not everyone cares to do this. I know when my oldest and his wife join us they get their own room. Even a villa would not give them the privacy they like and my son needs. For them, too much togetherness woudl be horrible.
You also want to look at budget for the families who will be with you. WE just came from a 9 day trip with free dining, 7 people at Old Key West, 2 bedroom villa. This was about $13,500 in total, so just about $2000 per person. A value would have been way less money, btu was not our choice.