There are several tour packages available.
Abercrombie & Kent is generally regarded as the better company to go with.
Their prices average about $3,000 for a 5 day trip.
Here is their website: http://www.abercrombiekent.com/travel-destinations/africa-safari/east-africa/uganda/
Adventure Trails is another good company:
Here is a description of their private 5 day trip. At the bottom of the page you will find a link where you may fill out a request for a cost quote based on several different parameters you set yourself: http://www.gorilla-safari.com/gorilla-safari.html
Here is a link to a page for other packages they offer:
Tours In Uganda offers lesser expensive trips at under $1,000 for a three day trip.
Here is a page where you can preview what they have to offer:
In almost all cases the prices you will be quoted DO NOT include air fair from the USA.