InfoPath Dev

I'm using IP 2007.  The cost Xpath is /my:expenseReport/my:items/my:item/my:cost The category menu XPath is /my:expenseReport/my:items/my:item/my:category One thing I didn't think to ask before: Do I need to create unique fields in my Data Source menu for each option in the drop down?

Unfortunately, I did not find my answer.  This is my first time ever using InfoPath, I really like using it, but things like XPath mean nothing to me right now.  Neither insturctions I followed helped me at all.  I've set each drop down menu choice with a unique value, but I don't know if that means anything or not.  I ...

I'm making an expense report and to make it easier on the Accounting Dept, I'd like to assign codes to certain drop down menu selections.  Is there anyway I can create a total based on the selection in the drop down menu and palce it total in a box seperate from any others?  In case I didn't explain it well here's an ...