Almost every airline has a webpage on charter services, google the name of the airline and charter services

United offers this warning

Customers interested in pursuing charters should be aware that the cost of exclusive occupancy can be substantially higher than scheduled service on United. We are able to provide customized routings and times, though weekend and off-peak travel will likely be lower priced than peak travel dates and times.

http://www.united.com/page/article/0,6722,51117,00.html

They also list the configurations of their different aircraft for number of passengers.

Contact both the charter and group sales and compare the prices. Some airlines for group sales will give you one free ticket for every so many paid seats so make sure to ask about that too.