This is a short tutorial on how to use Price and Contact fields as custom fields and use it only on several categories.
First of all please create custom fields with name 'price' and type inputbox.
This fields will be labeled on fields list
After this please go to DJ-Classifieds global configuration and in tab 'Global' change parameter "Use 'Price' " to 'Custom field'
Next assign this custom field to categories which you want to publish. Then this field Price will be published as custom field.
You can do the same trick with Contact extra field.