The Ultima Collectors Guide is a website maintained by Holger Bachert which has the various versions of the many Ultima games as a topic. It describes every version of each game in terms of its contents and the differences to the original version. Included are pictures of the boxes and the contents themselves.
There is also an "availability-meter" below each object, indicating how often it should be available on eBay and how much it could cost to buy there.
The site was last updated on October, 2009.