It's possible, it's just not best practice to do it the way that you are doing it now. You should change it to the following schema and you'll be much better off:
1) Create a secondary data source and then populate the drop-down by using this HowTo:
http://infopathdev.com/howto/tutorials/default.aspx?i=7f693c75c5c94604b01bdfd3a1edaedc2) Be sure to make the drop-down retain the ID value from the secondary list, and not the display name
3) If you want to show the display name in SharePoint create a new node in the same group that the drop-down is in and called it "DisplayName" or something, and populate it via instructrions found in this HowTo:
http://infopathdev.com/howto/tutorials/default.aspx?i=9111ef580f95479890fc18fb2c5bac854) Now you can perform whatever calculations you want with the mapping described in the secondary data source
I hope this helps, good luck!
Matt Faus / Microsoft InfoPath MVPQdabra® Software / Streamline data gathering to turn process into knowledge