Yesterday, the "I Am Hungry" Facebook application sold on eBay yesterday for $20,100.
The application has 250K+ installs and 150K users. That translates to $0.13 per user.
From the Ebay writeup:
What is "I am Hungry"
I am hungry is a very simple [Facebook] application: The user announces they are hungry by pressing a button. This puts a "hungry status" on their profile and on their news feed. If other friends are hungry within the hour, the user sees a list of them so they can meet up and eat together.
You can see the number of users who used the "I am hungry" feature to announce their hunger per day in the csv below.
What exactly are you selling?
You will get all the source code (with a complete copyright transfer to you), all database data associated with "I am hungry" (see details below), complete rights to the "I am hungry" application on facebook (you will be assigned as the "developer" of the application"), and 30 total hours of consulting to help you move the application to your servers, support you, and give you ideas for improvements.
More on the valuation from the AllFaceBook Unofficial blog.
Hattip: Ilya Vedrashko