collective.behavior.discount provides discount related behavior in addition to price behavior to dexterity content types.
For farther documentation about price behavior, refer to the collective.behavior.price.
The behavior can be added through the web or directly through the file system to the dexterity content type xml file like:
<property name="behaviors">
...
<element value="collective.behavior.discount.interfaces.IDiscount" />
...
</property>