How can I sum up fields that reside in multiple nodes?
The SUM functionality is actually built into Views for D7.

If you go to your Advanced settings and enable Aggregation, you will see the Aggregation settings applied to each field.

Click on 'Aggregation settings' for the field you'd like to do the SUM functions for and you can select 'SUM' from the 'Aggregation type' dropdown as shown in this screenshot



Taken from Source Sum a field